Optimum efficiency rotary machine having synchronous operation at a selectable speed

ABSTRACT

A machine includes a rotor with a magnetic field produced therein that is fixedly oriented with respect to the rotor. The rotor is rotatably mounted within a stator which has a plurality of windings. Means are provided for applying voltage pulses to the stator windings. When voltage pulses are applied to the stator windings, there is a torque due to the interaction of a magnetic field produced in the stator windings and the rotor magnetic field that causes the rotor to rotate. A control system responds to voltage induced across unpowered stator windings by the rotating rotor magnetic field and controls application of the voltage pulses to the stator windings. The voltage pulse sequence, frequency, and duration are determined by the control system. The control system may also adjust the voltage pulse amplitude. The voltage pulses are applied to the stator windings in such a way that the machine operates at optimum efficiency and at a selectable speed over a wide range of loads and/or line voltages. The control system includes means for plugging the stator windings in the event that the selectable speed is exceeded. This assures synchronous operation of the machine at the selectable speed. In the case of a machine with three or more phases, the control system additionally includes means for distinguishing between voltage induced across unpowered stator windings by the rotating rotor magnetic field and voltage due to switching transients and transformer action among the stator windings.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of a copending application ofJohn H. Knight and Milton S. Isaacson entitled ROTARY MACHINE, Ser. No.561,537, filed Mar. 24, 1975, now U.S. Pat. No. 4,027,215, issued May31, 1977, which is in turn a continuation-in-part of an application ofJohn H. Knight and Milton S. Isaacson entitled ROTARY MACHINE, Ser. No.484,563, filed July 1, 1974, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to the field of speed controls for rotarymachines and, particularly, to a speed control which utilizes pluggingto assure stability of a rotary machine at a selectable synchronousspeed over a wide range of loads and line voltages. Additionally, thisinvention relates to a control system for high speed rotary machineswith three or more phases, and, particularly, to a control system whichresponds to voltage induced across unpowered stator windings by arotating rotor magnetic field to control application of voltage pulsesto the stator windings.

Rotary machines have generally been classified as either DC or ACmachines. Both types of machines include means for generating at leasttwo different magnetic fields, one field being produced in a rotor andthe other field being produced in a stator. When these fields are notaligned with each other, a torque is established which brings about arotary movement of the rotor. As the rotor turns, the orientation of therotor field with respect to the stator field changes. As the fieldorientation changes, the magnitude of the torque on the rotor alsochanges so that the machine must include some means for reorienting onemagnetic field with respect to the other so as to maintain the torque onthe rotor near a maximum in order to sustain efficient operation.

In DC machines, the rotor armature normally has a plurality of windingswound thereon which are connected to an external power source throughbrushes and a mechanical commutator. The commutator causes voltage to beapplied selectively to the armature windings so that the magnetic fieldproduced in the rotor on the average will be aligned at a 90° angle tothe stator magnetic field produced in fixed stator windings or by apermanent magnet stator. Because the angle between the armature fieldand the stator field for a DC machine is on the average 90°, the DCmachine is generally efficient, and torque on the rotor is maximum.However, the DC machine does have a drawback over most AC machines. Whenthe DC machine is loaded by an external device connected to the rotor,the angular velocity of the rotor decreases in a somewhat linear fashionas the load increases. Consequently, DC machinery is not suitable foruse in applications where constant speed is required and the load and/orline voltage varies unless some form of auxiliary speed controller isutilized.

AC synchronous machines, on the other hand, are designed to utilize analternating current power source to power and synchronize the machine.The fixed frequency of the AC power source is automatically operative tocause rotation at a fixed angular velocity. When the synchronous machineis more heavily loaded, the machine is operative to cause the torqueangle, the angle between the rotor field and the stator field, to comeever closer to a 90° angle. In this manner, the torque on the rotorincreases as the load on the machine increases. Consequently, an ACsynchronous machine is able to maintain a constant speed even as theload varies. The problem with AC synchronous machines is that suchmachines do not operate at optimum efficiency for all loads. Indeed, theAC synchronous machine is most efficient when it is heavily loadedbecause the torque angle approaches 90°. When the AC synchronous machineis less heavily loaded, the torque angle is less than 90°, thus reducingthe machine efficiency.

In addition, AC synchronous machines do not have the ability to readilyadjust speed and are well known to have a tendency to be unstable. Thetorque angle of the synchronous machine is determined by the load. Ifthe load changes, the torque angle will change to accommodate the newload. As the AC synchronous machine changes toward a new torque angle,it will overshoot and then come back toward the new torque angle so asto oscillate into the new torque angle. An AC synchronous machine,however, depending on the amount and rate of load change, can oscillateso badly that it will drop out of synchronism unless means such asdamper windings are provided for reducing oscillation.

The rotary machine which is described in Ser. No. 561,537 filed Mar. 24,1975 incorporates the desirable features of both the DC commutatormachine and the AC synchronous machine. Specifically, the rotary machineoperates efficiently as a DC commutator machine and also operates at aselectable synchronous speed over a wide range of loads and/or linevoltages as an AC synchronous machine. Moreover, the rotary machine doesnot have the drawbacks of other DC brushless motors or transducerlessmachines since operation of the rotary machine control system issubstantially independent of machine size.

Although the rotary machine which is described in aforementioned Ser.No. 561,537, in contradistinction to an AC synchronous machine, operatesas a DC commutator machine because it operates at a constant torqueangle, due to the fact that the rotary machine is preferably maintainedat a selectable speed if the load changes, it has a tendency to becomeunstable as in the case of an AC synchronous machine. That is, if theload changes, the voltage pulse amplitude and/or width will change toaccommodate the new load. If the load changes rapidly by a substantialamount, however, the voltage pulse amplitude and/or width that isnecessary to maintain the rotary machine at the selectable speed can beovershot, then undershot, and then overshot again, etc. so that therotary machine oscillates and possibly drops out of synchronism.

In the preferred form of the rotary machine in aforementioned Ser. No.561,537, the control system responds to voltage induced across unpoweredstator windings by the rotating rotor magnetic field to control theapplication of voltage pulses to the stator windings. When such amachine with three or more phases is operated at high speed, however,transformer action among the stator windings which occurs at times whenvoltage pulses are applied to and removed from stator windings affectsthe voltage across unpowered stator windings. This in turn can causeimproper control of the application of voltage pulses to the statorwindings.

OBJECTIVES OF THE INVENTION

An objective of the invention is to provide means for synchronizing amachine upon changes in load or line voltage to thereby maintainsynchronous operation at a selectable speed.

An additional objective of the invention is to provide means forsynchronizing the machine at a selectable speed during start-up and/orfor synchronizing the machine at a different selectable speed duringoperation.

Another objective of the invention is to provide means for preventingunstable oscillation of the machine speed to thereby assure synchronousoperation of the machine at a selectable speed.

It is also an objective of the invention to provide a machine which isstable and which runs as efficiently as a DC commutator machine, has theability to be operated synchronously over a wide range of machinetorques, and has a design which is substantially independent of machinesize.

It is another objective of the invention to provide a machine which runssynchronously and efficiently at a selectable speed while the load orline voltage varies by providing means for varying the duration ofvoltage pulses applied to the stator windings to compensate for smallline voltage and/or load variations and by providing means for alteringthe voltage pulse sequence and/or for adjusting the voltage pulseamplitude to compensate for large line voltage and/or load variations.

It is a further objective of the invention to provide a machine whichresponds to induced voltage in unpowered stator windings to controlvoltage pulses applied to the stator windings so that the machine runsas a controlled DC machine at a selectable speed over a wide range ofmachine torques.

It is another objective of the invention to provide a machine which runsefficiently and synchronously at a selectable speed by providing meansincluding a pulse generator for producing pulses derived from the zerocrossings of the induced voltage in conjunction with a referenceoscillator for detecting deviation from the synchronous speed andcorrecting the machine operation to compensate for the deviation.

It is yet another objective of the invention to provide a high speedmachine with three or more phases which runs efficiently andsynchronously at a selectable speed by providing means to utilize thezero crossings of induced voltage in unpowered stator windings andignore the zero crossings of voltage across stator windings associatedwith power switching and transformer action among the stator windings tothereby provide signals indicative of the rotor position for controllingthe switching of power from one stator winding to another.

SUMMARY OF THE INVENTION

One aspect of the invention is predicated on the concept of "plugging"when a selectable synchronous speed is exceeded, that is, changing thesequence in which voltage pulses are applied to the stator windings of amotor when the motor is overspeed. By changing the sequence in whichvoltage pulses are applied to the stator windings, the speed of therotor is retarded so as to synchronize the motor speed at the selectablesynchronous speed. The use of plugging synchronizes the motor speed uponchanges in load or line voltage, during start-up, or when the selectablesynchronous speed is reset during operation. Plugging prevents unstableoscillation of the motor speed and assures synchronous operation.

A further aspect of the invention is predicated on the concept ofdistinguishing between voltage induced across unpowered stator windingsby the rotating rotor magnetic field and voltage due to switchingtransients and transformer action among the stator windings. Thispermits utilization of the induced voltage to determine the precise timewhen voltage pulses should be applied to the stator windings so as tomaintain constancy of average torque angle for optimum efficiency andalso to maintain constant speed over a wide range of motor torques.

In a preferred embodiment of the invention, a rotor is provided with amagnetic field produced therein that is fixedly oriented with respect tothe rotor. The rotor freely rotates within a stator located around therotor, the stator having a plurality of windings arranged so thatvoltage applied to the stator windings produces a magnetic field whichinteracts with the rotor magnetic field to produce a torque on the rotorcausing rotation thereof. As the rotor turns, voltage is induced acrossthe stator windings. A control network responds to the voltage inducedin unpowered stator windings, and, particularly, to the zero crossingsof the induced voltage, to activate a power network which normallyproduces voltage pulses that are applied to the stator windings at apredetermined frequency in a predetermined sequence. These voltagepulses cause the rotor to rotate at the selected speed in apredetermined direction of rotation. The voltage pulses are produced attimes when the torque angle is on the average a selectable value, theselectable value being 90° when maximum efficiency is desired.

If the rotor speed increases, a control network senses this conditionand causes the duration of the voltage pulses applied to the statorwindings to be reduced, thereby causing the rotor speed to decrease. Ifthe rotor speed decreases, the voltage pulses applied to the statorwindings are widened by the control network, thus increasing rotorspeed. The voltage pulse narrowing and widening compensates forrelatively small speed variations caused by changes in load and/or linevoltage.

For large speed increases, the control network "plugs", that is, altersthe normal sequence of the voltage pulses applied to the statorwindings. The control network may also decrease the voltage pulseamplitude in the event of a large increase in speed and/or increase thevoltage pulse amplitude in the event of a large decrease in speed. Thedecrease or increase of voltage pulse amplitude besides causing therotor to slow down or speed up also allows the pulse width to remainwithin selected limits. These voltage pulse sequence and amplitudechanges maintain synchronous operation when large machine torquevariations occur.

In the case of a motor with three or more phases, the control networkincludes means for distinguishing between voltage induced acrossunpowered stator windings by the rotating rotor magnetic field andvoltage due to switching transients and transformer action among thestator windings. Slope sensors are preferably included to cause thecontrol network to respond only to the induced voltage and,particularly, to zero crossings of the induced voltage. Use of the slopesensors facilitates detection of the zero crossings of the inducedvoltage no matter what the motor speed.

DESCRIPTION OF THE DRAWING

The foregoing and other objectives, features, and advantages of thisinvention will become more clear from the following detailed descriptionof a preferred embodiment therefor taken in connection with the drawingin which:

FIG. 1 is a schematic diagram of a two phase machine connected to powercircuitry and control circuitry;

FIG. 2 is a timing chart showing voltages at different times during thenormal operation of the machine in FIG. 1;

FIG. 3 is a speed-torque curve for the machine in FIG. 1;

FIG. 4 is a schematic block diagram which includes the speed control ofthe invention;

FIG. 4a is another schematic block diagram showing the function of eachmajor element in FIG. 4;

FIG. 5 is a schematic diagram for a motor power supply which willprovide different voltage outputs in response to increase or decreasesignals;

FIG. 6 is a detailed diagram showing the voltage across a stator windingduring the period of time when the winding changes from powered tounpowered;

FIGS. 7-11 comprise a detailed circuit diagram showing oneimplementation of the control circuitry of the invention;

FIG. 12 is a schematic diagram of a three phase/four wire machine;

FIG. 13 shows a timing diagram for a three phase/four wire machine whilethe machine is operating without pulse width control;

FIG. 14 shows a schematic diagram of a circuit for accepting positivepulses from a plurality of zero crossing detectors and utilizing thesepulses to produce control pulses for the up/down counters of acommutation circuit;

FIG. 15 is a circuit diagram for an alternative machine starting circuitespecially useful for machines with a large inertia;

FIG. 16 is a timing chart for the circuit of FIG. 15;

FIG. 17 shows voltages across the stator windings of the machine in FIG.1 in the event of an overspeed condition and demonstrates speed controlby means of plugging; and

FIG. 18 is a circuit diagram for a slope sensor.

GENERAL DESCRIPTION

As shown in FIG. 4a, a motor 10 includes a fixed magnet rotor 11 and astator 12 having two windings shown as the A coil and B coil. A motorpower supply 13 is connected to the A coil and B coil for providingvoltage pulses to the A coil and B coil. A stepping switch 14 isconnected to the motor power supply 13 via a gate circuit 15 to controlwhich stator winding receives electrical power at any given moment oftime. Normally, the inhibit signal to the gate circuit 15 is not presentso that the position of stepping switch 14 indicates to the motor powersupply 13 the stator winding to which voltage is applied.

When the rotor 11 of the motor is rotating, the magnetic field of therotor induces a voltage across the stator windings. Zero crossingdetectors 16 are connected to the stator windings to detect when theinduced voltage in either stator winding passes through zero volts. Whenthis occurs, a position pulse is generated at a time when the rotor isat a predetermined position which, for the two stator winding machineshown, corresponds to the time when the torque angle is 90°. In otherwords, the position pulse is generated at a time when the magnetic fieldof the rotor is arranged at a 90° angle to the magnetic field generatedby whichever stator winding is receiving power. Since the position pulseoccurs at a point in time when the torque angle is 90° for the machineshown, the invention contemplates power pulse symmetry about this timeso as to maximize efficiency.

The position pulses generated at the output of the zero crossingdetectors 16 are input to a commutation circuit 17 and are utilized bytwo counters 18 and 19 that alternately count up at a rate of f/2 forthe time period between successive position pulses and alternately countdown at a rate of f. These counters, when they count down to zero,generate a borrow pulse or second position pulse at their output. Sincethe counters 18 and 19, when they are counting up, count for a period oftime corresponding to rotation of the rotor of 90° for the two phasemachine described, the borrow pulse or second position pulse isgenerated at a time after the counter starts counting down whichcorresponds to the time required for the rotor to rotate 45° after aposition pulse occurs. As hereinafter mentioned, the borrow pulse orsecond position pulse occurs at a time at which power should be shiftedfrom one stator winding to another assuming that power is applied toeach stator winding for the period of time required to rotate the rotor90° at a selected rotor speed.

The machine includes a rate variable oscillator 21 producing four pulsesevenly spaced in time during the time period required for the rotor tomake one complete revolution at the desired speed. These pulses set aspeed latch 22.

The borrow pulses or second position pulses generated by the counters 18and 19 are utilized to set a logic latch 23. When the logic latch 23 andthe speed latch 22 are set, an AND gate 24 connected to the outputs ofthe logic latch and the speed latch is operative to generate a signal atits output for stepping the stepping switch 14 so as to remove powerfrom one stator winding and apply it to another stator winding inaccordance with the desired sequence so as to maintain rotation of therotor.

In normal operation of the machine, the logic latch 23 will be set at atime prior to the setting of the speed latch 22. A pulse centeringcircuit 25 is provided to respond to the setting of the logic latch tostart a counter for counting pulses at a rate of f for the time periodbetween the setting of the logic latch and the setting of the speedlatch. The number counted by this counter represents the amount of timeby which a power pulse applied to a given stator winding of the machineis shortened at its leading edge by the circuitry previously described.The objective of the pulse centering circuitry is to shorten thetrailing edge of the same power pulse so as to center the pulse about atime when the average torque angle is a predetermined value which, formaximum efficiency, corresponds to an average torque angle of 90°. Thepulse centering circuit compares the number in its counter continuallywith the counter which is counting down at a rate of f. When these twocounters are equal, a comparator in the pulse centering circuitgenerates an inhibit signal which is operative to close the gate circuitthereby causing the power supply to remove power from the powered statorwindings. As such, the trailing edge of a power pulse applied to astator winding is shortened by the same period of time that the leadingedge of the same power pulse was delayed.

It will be readily recognized by those of skill in the art that thecircuitry heretofore described in the general description is operativeto control the operation of the machine so as to maintain a constantselectable speed determined by the rate variable oscillator 21 evenwhile the load torque on the rotor varies and also to maintain operationat an optimum efficiency which preferably is maximized. In other words,by widening or narrowing power pulses applied to the motor statorwindings, the speed of the rotor can be maintained at a constantselectable value even while the load torque varies. Additionally, thepower pulses applied to the motor stator windings are appliedsymmetrically so that the average torque angle will be a selectablevalue which is preferably 90° and, therefore, the motor operates atmaximum efficiency.

The count of the counter in the pulse centering circuit is correlated tothe pulse width of power pulses applied by the power supply to thestator windings. When the count becomes relatively small, the pulsewidth is relatively large and, conversely, when the count is large, thepulse width is relatively small. As the load torque on the rotorchanges, the width of the power pulses applied to the stator windingswill also change so as to maintain synchronous speed. Generally, as theload torque increases, the pulse width will increase and, conversely, asthe load torque decreases, the pulse width will decrease. It has beendiscovered, however, that pulse width control for machines of the typegenerally described above is operative to adjust the machine operatingconditions sufficiently to compensate for relatively small load torquevariations. However, for large load torque and line voltage variations,pulse width control is not effective for maintaining synchronousoperation.

In order to maintain synchronous operation when large load torquevariations occur, an amplitude control circuit 26 is provided whichresponds to the pulse centering circuit 25. If the pulse width exceeds apredetermined upper limit, the amplitude control circuit 26 generates asignal at its output which is transmitted to the power supply 13 toincrease the amplitude of power pulses applied to the stator windings.On the other hand, if the pulse width falls below a predeterminedminimum value, the amplitude control circuit 26 transmits a signal tothe power supply 13 to decrease the amplitude of power pulses applied tothe stator windings. In order to maintain synchronous operation whenlarge load and/or line voltage variations occur which increase the speedof the rotor above the selectable speed established by the rate variableoscillator 21, the plugging circuit 9 synchronizes the motor 10 to theselectable speed. The plugging circuit 9 also brings the motor 10 intosynchronism during start-up, speed increase or decrease due to manualreset of the rate variable oscillator 21, etc. and thereafter maintainssynchronism at a constant selectable speed.

The plugging circuit 9 is responsive to the position pulses from thezero crossing detectors 16 and to the position of the stepping switch 14to sense a position pulse that occurs prematurely thereby indicating anoverspeed condition. If a position pulse occurs prematurely, theplugging circuit 9 causes the motor power supply 13 to apply a powerpulse to a stator winding to plug the stator winding until such time asthe commutation circuit 17 causes the stepping switch 14 to changeposition. Consequently, the rotor speed is momentarily retarded so as tosynchronize the motor 10 at the selectable speed.

The use of plugging provides a highly responsive method to correct anoverspeed condition. The plugging method of overspeed correctionprevents oscillation of the motor speed and thereby assures synchronousoperation of the motor 10 at the preset speed. In any event, thevariation of the pulse amplitude to the stator windings is operativewith the pulse widening and shortening and plugging circuitry tomaintain synchronous operation even when a large load torque or linevoltage variation occurs.

It will become more clear later than the principles of the invention asdescribed generally above are equally applicable to machines havingthree or more stator windings or phases. Indeed, the principles of theinvention are applicable to machines of any size having any number ofstator windings or phases.

DETAILED DESCRIPTION

Referring now to FIG. 1, the motor 10 of this invention is characterizedby having a rotor, shown diagrammatically as 11, and a stator, showndiagrammatically as 12. The rotor 11 is of the stationary field type andhas some means associated therewith for producing a magnetic field orfields therein with a fixed direction and orientation with respect tothe rotor 11, as indicated generally by the arrow 30. The rotor fielddirection is generally aligned in a direction perpendicular to the axisof rotation of the rotor 11 which, for the machine showndiagrammatically in FIG. 1, is perpendicular to the sheet. In somemachines, however, the rotor field may be different from that shown inFIG. 1 as, for example, in machines having multiple rotors or othermachines where the rotor field is not aligned perpendicular to the axisof rotor rotation.

The rotor field producing means may comprise a permanent magnet mountedin the rotor itself. Alternatively, the rotor field may be generated bycurrent carrying windings on the rotor which produce the desired rotorfield fixedly oriented with respect to the rotor when current flowstherethrough. The rotor windings are electrically connected by a slipring and some form of brush connection or the like to an external sourceof electrical power. A further alternative means of generating themagnetic field in the rotor 11 is to externally induce the magneticfield by any suitable means.

As with most machines, the stator 12 includes a plurality of statorwindings shown diagrammatically at 31 and 32 which were referred toabove as the A coil and B coil, respectively. Each stator winding 31 and32 comprises a plurality of turns of electrically conductive insulatedwire wound in a manner well known in the electrical machine art. Whencurrent flows through either of the stator windings 31 or 32, a magneticfield is produced thereby having a direction which is also perpendicularto the axis of rotation of the rotor 11. So long as the direction of thefield in either stator winding 31 or 32 is not the same as the directionof the magnetic field of the rotor 11, a torque is exerted on the freelyrotatable rotor 11 that causes the rotor 11 to turn about its axis ofrotation.

The torque on the rotor 11 is a function of the torque angle, that is,the physical angle between the direction of the stator field and thedirection of the rotor field. When the stator field is constant and thetorque angle is 90°, the force on the rotor is maximum. When the torqueangle is 0°, the force is zero. For the machine in FIG. 1, if a voltageis applied to the stator winding 31 so as to produce a magnetic fieldshown generally by the arrow labeled φA, a force will be exerted on therotor 11 to cause it to rotate in a clockwise direction. As the rotor 11rotates in the clockwise direction from the position shown, however, theforce exerted on the rotor 11 becomes smaller and smaller as the rotor11 rotates toward a position where the rotor field becomes aligned withthe stator field. The power must be shifted to winding 32 in order forrotation to be continued. As such, some form of field commutation isnecessary in order to sustain rotary motion of the rotor 11.

The stator field commutation is accomplished by sequentially controllingthe application of voltage to the stator windings 31 and 32. A controlcircuit, schematically shown at 33 in FIG. 1, responds to the voltageinduced in the stator windings 31 and 32 by the rotating rotor toactivate the power supply circuit 13 to produce power pulses which areapplied to the stator windings 31 and 32 in a predetermined sequence ata predetermined selectable frequency to maintain rotation of the rotor11 at a constant speed even when load torque changes.

The desired predetermined sequence of power pulses for clockwiserotation of rotor 11 is shown diagrammatically in the upper half of FIG.2. The rectangular areas 34, 35, 36, and 37 each represent the periodsof time during which electrical pulses having a fixed amplitude areapplied to a stator winding. The area 34, for example, corresponds to aperiod of time when a voltage of +V is applied to the stator winding 31.The area 36, on the other hand, corresponds to a period of time when avoltage of -V is applied to the same winding 31. Similarly, therectangular area 35 corresponds to a period of time when a pulse havinga voltage of +V is applied to the stator winding 32. The rectangulararea 37 corresponds to the period of time when a pulse having a voltageof -V is applied to the stator winding 32.

The sequence and time at which pulses are applied to the stator windings31 and 32, as shown in FIG. 2, is important in defining the direction ofrotation and machine efficiency. To completely comprehend thisstatement, however, the basic machine operation must be understood. Therotor position shown in FIG. 1 is arbitrarily defined as the 90°position. When at this position, the direction of the rotor fieldindicated by the arrow 30 is disposed at a 90° angle to the direction ofthe field which is generated in the A coil 31 when voltage is appliedthereto. Also, when the rotor is at the 90° position, the torque on therotor 11 due to interaction of the rotor field and the field in the Acoil is maximum. In order to maximize machine efficiency, the A coil andB coil must be energized for time periods during which the averagetorque angle is 90°. By way of example, power should be applied to the Acoil during the period of time in which the rotor 11 rotates from aposition where the rotor magnetic field direction 30 is oriented 45°counterclockwise to a position where the rotor field is 45° clockwise ofthat position shown for the rotor 11 in FIG. 1. As such, the averagetorque angle, when power is applied to the A coil for the period of timewhen the rotor rotates from a position 45° counterclockwise of thatshown in FIG. 1 to a position 45° clockwise of the position shown inFIG. 1 is 90°, and the machine, when so operated, runs most efficiently.The pulse width of power pulses applied to stator windings can be variedbut, so long as the leading edge and the trailing edge of the pulse aresymmetrically timed, so that the pulse is centered at the 90° position,as shown, the average torque angle is 90°, and maximum efficiency ismaintained.

Once the rotor 11 rotates 45° in the clockwise direction from theposition shown in FIG. 1 to its 135° position as shown by the conventionestablished in FIG. 2, the torque angle between the rotor field and thefield in the A coil is 45°. At this position, the torque on the rotor 11is significantly reduced from that at the 90° torque angle position.Consequently, it is desirable to turn off the power to the A coil andturn on the power to the B coil. The control circuit 33 accomplishesthis switching function by first detecting the position of the rotor 11with respect to the fixed stator in a manner which will be described ingreater detail later and then issuing control signals to the powercircuit 13 when the rotor 11 reaches the desired position for switchingpower from one winding to the other. Consequently, when the rotorreaches its 135° position, the power circuit 13 switches power from theA coil to the B coil. This change of power from the A coil to the B coilis shown in FIG. 2 at the 135° position whereat the power pulse 34 isturned off and the power pulse 35 is turned on.

The B coil remains powered for the period of time while the rotor movesfrom its 135° to its 225° position. After the rotor 11 has reached its225° position, power must again be applied to the A coil, although thefield generated thereby must be in a direction opposite to thatgenerated by the pulse shown by the rectangular area 34 in FIG. 2 ifrotary motion is to be sustained. This power pulse is shown generally bythe rectangular area labeled 36 which produces a magnetic field in the Acoil having a direction opposite to that generated by the pulseindicated by the rectangular area 34. The control circuit 33 assuresthat this pulse 36 is applied to the A coil for the period of timerequired for the rotor 11 to rotate from its 225° to its 315° position.

When the rotor 11 reaches its 315° position, the control circuit 33removes power from the A coil and again applies power to the B coil asindicated generally by the rectangular area 37 in FIG. 2. Power isapplied to the B coil for the period of time in which the rotor 11rotates from its 315° position to its 45° position.

The control circuit 33 is operative to cause the power circuitry 13 toproduce power pulses indicated by the rectangular areas 34, 35, 36, and37 in the sequence shown. Thus, so long as the power pulse sequence ismaintained, the rotor will rotate continuously. Further, as will becomemore clear later, by setting the maximum frequency of power pulsegeneration, the control circuit 33 can select the maximum speed of themotor.

The machine and its mode of operation as described above has aspeed-torque characteristic curve like that shown in FIG. 3. The no loadspeed of the machine shown in FIG. 1 is indicated in FIG. 3 generally at38. This no load speed is determined by the setting for the maximumpermissible frequency of the power pulses applied to the stator windings31 and 32. The higher the frequency of the power pulses, the higher theno load speed will be and vice versa.

When a load is applied, however, the rotary speed will not decreasebelow the no load speed shown generally at 38 unless the load applied isgreater than Tm, the load at which the machine no longer operatessynchronously. As the load increases above Tm, the speed of rotationdrops just as a DC machine would operate when the load is increased.

The foregoing is a very general description of the operation of themachine shown diagrammatically in FIG. 1. The description indicates thatpower is applied to each stator winding for a period of time whichcorresponds to the time required for the rotor 11 to rotate 90°. Inactuality, for the preferred machine shown in FIG. 1, power is appliedto the stator windings 31 and 32 for a period of time which is less thanthat shown in FIG. 2 if the load torque on the machine is less than themaximum torque Tm as shown in FIG. 3.

In the preferred embodiment, the control circuit 33 is operative toapply power to the stator windings for varying periods of time,preferably centered about the 90° position depending on the load torqueitself. For example, if the load torque increases, the rotorinstantaneous velocity tends to decrease. The control circuit 33 sensesthis condition and responds by widening the power pulses applied to thestator windings. When the load torque falls, the instantaneous speed ofthe machine tends to rise so the control circuit 33 responds by reducingthe power pulse width thereby maintaining an average 90° torque angleand constant speed operation.

The control circuit 33 also operates to maintain constant machineefficiency. This is accomplished by centering all power pulses, whetherwidened or shortened, at a time when the average torque angle is aconstant value. For maximum efficiency, the power pulses are centered sothat the average torque angle is 90°. For lower efficiency, the powerpulses are centered so that the average torque angle is less than 90°.

While the control circuit 33 is capable of varying the pulse width overa wide range, it is desirable to prevent the pulse width from becomingtoo small because small pulse widths would give rise to very unevenmotor torque generation during each revolution of the rotor. As such, itis desirable to arbitrarily select the minimum pulse width to be 65°,i.e., the time required for the rotor to rotate 65° at the selectedspeed as shown, for example, at A in FIG. 2. Also, for reasons that willbecome clearer later, it is desirable to arbitrarily select a maximumpower pulse width of 85°, i.e., the time for the rotor to rotate 85° atthe selected speed as shown at B in FIG. 2. The foregoing limits,however, have been arbitrarily selected and other limits may be utilizedwithout substantially affecting the machine operation. When these limitsare established, the degree to which pulse widening and shortening cancompensate for varying load torques is reduced.

Since it is desirable to have the machine run at a constant speed evenwhen there are wide variations in the load torque and also because pulsewidening and shortening is able to compensate only for small load torquevariations because the pulse width is not permitted to vary widely andhas only a limited amount of power variation, the power circuitry 13under the control of the control circuitry 33 is operative to vary theamplitude of the voltage of power pulses to stator windings to permitcompensation for large variations in load torque. For example, shouldthe control circuit 33 determine that the power pulse duration exceeds85°, i.e., the time required for the rotor to turn 85° at the selectedspeed, this condition is detected by the control circuitry 33 whichnotifies the power circuitry 13. The power circuitry 13 responds tonotification from the control circuitry 33 by incrementally increasingthe voltage of the power pulse applied to the stator windings. Thisincremental increase in the voltage is operative to increase thegenerated torque to thereby compensate for the increased load torque onthe machine. On the other hand, should the control circuit 33 determinethat the power pulse duration is shorter than 65°, i.e., the timerequired for the rotor to turn 65° at the selected speed, the powercircuitry 13 responds to the control circuitry 33 by incrementallyreducing the voltage of the power pulse applied to the stator windings.Consequently, the generated torque is reduced to compensate for reducedload torque and the torque angle is maintained at an average of 90° atthe selected constant speed. Since the power circuitry 13 is operable toincrementally vary the power pulse voltage applied to the statorwindings 31 and 32 over a wide range of voltages, the generated torqueof the motor can vary widely to compensate for large load torquevariations while the motor operates at a selected constant speed. Assuch, the machine of the invention can operate at a constant selectablespeed while the load torque varies over a wide range.

If an overspeed condition develops, such as might occur, as will bedescribed, during start-up or a manual increase in the selectable speedor such as might be occasioned by a manual decrease in the selectablespeed or sudden removal of a load or an overvoltage condition of theline voltage, the control circuit 33 causes the power circuit 13 to plugthe stator windings so that the rotor rotation will be retarded and themotor 10 becomes synchronous at set speed. As will be made clear later,plugging provides a highly responsive method for correction of anoverspeed condition and prevents oscillation of the motor speed whichmight otherwise result in loss of or inability to achieve synchronousoperation. That is, plugging results in the motor 10 having very stable,synchronous operation.

FIG. 4 is a schematic block diagram according to the invention for theelectrical circuit including the control circuitry 33 and the powercircuitry 13, as described generally in FIGS. 1 and 4a, to control amachine of the type shown generally in FIG. 1.

The circuitry includes a start circuit 40, the electronic steppingswitch 14, the motor power supply 13, the zero crossing detector 16, thecommutation circuit 17, the pulse centering circuit 25, the voltageamplitude control circuit 26, and the plugging circuit 9. These elementsof the circuit will be discussed in detail below in separate identifiedsections.

Start Circuit

The start circuit 40 is operative to start rotation of the machine in adesired direction when power is turned on. The start circuit 40 includesa pulse generator such as a single shot timer which produces a singlepulse at its output 41. This pulse comprises one input to an OR gate 42.The pulse passes through the OR gate 42 to become an input to theelectronic stepping switch 14 and also an input to the start circuit 40itself via a reset wire 43. The pulse on the reset wire 43 resets thestart circuit 40 by, for example, turning off the single shot timer. Thetimer then begins another timing cycle and produces another pulse aftera predetermined period of time. In this manner, the start circuit 40 incombination with the OR gate 42 produces a series of pulses at the inputof the electronic stepping switch 14. The pulse rate is selectable andmay vary from machine to machine but typically is in the order of 100pulses per second for small machines and lower in frequency for largermachines.

The start circuit may be modified so as to provide starting pulses at avarying frequency. By producing start pulses first at a low frequencyand gradually increasing the pulse frequency, high inertia machines aremore easily started than by using a start circuit which generates startpulses at one fixed frequency. A variable frequency oscillator may beutilized to produce pulses initially at a low rate and graduallyincreasing the pulse rate until the rotor speed is sufficient to inducea detectable voltage in the stator windings. Once normal circuitoperation takes over, the start circuit is continually reset by thenormal pulse circuitry.

An alternative starting circuit is shown in FIG. 15. This circuitproduces a series of pulses at its output point 44 first at a slow rateand then at a successively increasing rate as shown by the timing chartof FIG. 16. After 16 time periods, the process is repeated.

If the circuit of FIG. 15 is connected as shown to the actual systemcircuitry of FIG. 8, however, the 82K resistor 45 in the start circuitshown in FIG. 8 must be removed to prevent pulse generation but allowevery pulse on DD to generate a pulse at AD. As such, the controlcircuitry is disabled until the up/down counters produce borrow pulsesindicating the rotor is rotating fast enough for the control circuitryto take over control.

Other approaches for producing start pulses which vary in frequency areavailable. The start pulses are generated by any suitable pulsegenerator that produces pulses at a varying frequency and connected topoint 44 on FIG. 15. The start circuit is disabled when borrow pulses,also referred to as second position pulses, are generated by the up/downcounters.

ELECTRONIC STEPPING SWITCH

The stepping switch 14 in FIG. 4 is simply a counter circuit or the likewhich sequentially activates one output 47, 48, 49, or 50 in response toan input pulse received from the OR gate 42. For the preferred system,the switch 14 first activates output 47 in response to the first pulsereceived from the OR gate 42. Successive pulses received from the ORgate 42 activate stepping switch outputs 48, 49, and 50 in sequence.Further OR gate 42 output pulses cause the stepping switch 14 toactivate its outputs 47, 48, 49, and 50 in a repetitive sequence.Activation of the outputs 47-50 in sequence is operative, as will becomeclearer later, to cause the rotor of a motor like that shown in FIG. 1to rotate in one direction.

A switch 51 is provided for reversing the stepping switch 14 sequency,i.e., the sequence in which outputs 47, 48, 49, and 50 are activated.When the switch 51 is open circuited as shown in FIG. 4, the outputsequence is as described above. However, when the switch 51 is closed,the sequence is reversed. That is, the outputs are activated in reverseorder so that the output activation sequence is 50, 49, 48, 47. Thisreverse sequence, as will become more clear later, will cause the rotorof the motor to rotate in the reverse direction from that for theearlier described sequence because power is applied to the statorwindings in reverse sequence from that described earlier.

Each output 47-50 is connected to one input of an AND gate 52, 53, 54,or 55 respectively of the gate circuit 15. The second input to each ANDgate 52, 53, 54, and 55 is connected to a common inhibit wire 56. Thesignal normally on the inhibit wire 56 is operative to let the signalson the wires 47, 48, 49, and 50 determine which AND gate 52, 53, 54, or55 has an active output. An inhibit signal on the inhibit wire 56 isoperative to deactivate all the AND gates 52, 53, 54, and 55 so as tomake all the outputs thereof inactive. The inhibit signal on wire 56 isdeveloped in a manner to be described in greater detail later.

Motor Power Supply

The output of AND gate 52 is connected to one input of an associated ORgate 68. Similarly, the outputs of AND gates 53, 54, and 55 areconnected to one input of associated OR gates 69, 71, and 72,respectively. The OR gates 68, 69, 71, and 72 each produce controlsignals at their respective outputs 58, 59, 60, and 61 which control themotor power supply 13. The motor power supply 13 has outputs labeled +V,A coil, GROUND, B coil, and -V. The +V and -V outputs are referencevoltage outputs which are used by the zero crossing detector 16 whichwill be described in greater detail later. The GROUND output is a groundconnection. The outputs labeled A coil and B coil are each connected toone end of the A coil and the B coil of a motor shown schematically inFIG. 1. The other end of the A coil and B coil is connected to ground.

The stator winding to which power may be applied is controlled by inputcontrol signals on lines 58-61 to the motor power supply 13. When aninput control signal is on line 58, the motor power supply 13 respondsby placing a positive voltage on the output wire labeled A coil. When aninput control signal is on line 59, the motor power supply 13 respondsto place a positive voltage on the line labeled B coil. Likewise, whenan input control signal is on line 60, the motor power supply 13responds to place a negative voltage on the output line labeled A coil.In a similar manner, when an input control signal is on line 61, themotor power supply 13 responds to place a negative voltage on the outputline labeled B coil.

The motor power supply 13 may comprise any conventional power supplycapable of generating an output signal having both a positive and anegative voltage which is sufficient to power the connected machine. Aswitching mechanism is provided in the motor power supply 13 to respondto control signals on lines 58-61 so as to apply power signals to eitherthe A coil or the B coil output wires in accordance with the inputcontrol signal. This switching mechanism can be a relay or electronicswitching network responsive to the input control signals on lines 58,59, 60, and 61.

The circuit diagram shown in FIG. 5 is one possible power supplyconfiguration for performing the functions described above in connectionwith the motor power supply 13. The circuit shown in FIG. 5 includes twoidentical power transformers 64 and 65 each with multiple tappedsecondary coils 66 and 67 respectively, i.e., each secondary coil taphas the same voltage as the corresponding tap on the other secondarycoil. Additionally, the voltage difference between any two adjacent tapsof either transformer 64 or 65 is preferably the same, thus makingpossible incremental voltage increases at the power supply output. Theoutput taps of each transformer 64 and 65 are connected to a steppingswitch 73 and 74, respectively, each of which is actuated by a commonactuator 75. The stepping switches 73 and 74 are each connectedelectrically to one output tap of the power transformer 64 and 65respectively so that the voltage between the connected tap and groundfor each stepping switch 73 and 74 is identical. The actuator 75 isoperative in response to receipt of pulses on either the Up or Downinput thereto to simultaneously switch each stepping switch 73 and 74either in a clockwise or a counterclockwise direction. Clockwiserotation of the stepping switches 73 and 74 is operative toincrementally increase the magnitude of the voltage output at points 76and 77 while counterclockwise rotation thereof is operative toincrementally reduce the magnitude of the voltage output at these points76 and 77. The stepping switches 73 and 74 should include a mechanicalstop to prevent either clockwise or counterclockwise rotation thereof tobeyond the highest and lowest voltage output taps of the powertransformers 64 and 65.

The output point 76 for the stepping switch 73 is connected to arectifier and filtering network shown generally at 78 which is operativeto produce at its output a positive DC voltage +V. The output point 77for the stepping switch 74 is connected to a rectifying and filteringnetwork 79 so as to produce a negative DC voltage -V at its output. Thepositive output of the rectifying network 78 is connected to the movablearm of relays R1 and R2. The negative output of the rectifier andfiltering network 79 is connected to a movable arm of relays R3 and R4.Each of these relays R1, R2, R3, and R4 are of the single pole/singlethrow type with the relay contact being normally open. The outputcontacts of relays R1 and R3 are connected to the A coil output whilethe output contacts of relays R2 and R4 are connected to the B coiloutput. The relays R1, R2, R3, and R4 are respectively energized byinput pulses on lines 58, 59, 60, and 61 from the OR gates 68, 69, 71,and 72 shown in FIG. 4. Consequently, signals on lines 58-61 areoperative to connect the A coil and B coil outputs to either thepositive or the negative voltage generated by the power supply in amanner previously described.

While the foregoing description of the motor power supply 13 andespecially the circuitry shown in FIG. 5 has been directed to aparticular embodiment therefor, it will be recognized by those skilledin the art that numerous modifications may be made to this circuit whilestill providing the same function. It is particularly clear that theswitching functions which are performed by mechanical switches in thecircuit shown in FIG. 5 can easily be replaced by electronic circuitryfor accomplishing the same result. Furthermore, other circuitry may beutilized to provide a power supply having different selectable voltageoutputs. For example, rotating machinery, magnetic, electronic, ormechanical regulators may be used. One such regulator is a switchingregulator. Switching regulators designed according to the description inthe publication entitled "Designing Switching Regulators" dated March,1969 and distributed by the National Semiconductor Company are suitablefor use in the motor power supply of the invention. A single supply withappropriate switches for reversing current in the motor windings mayalso be used. A single supply with appropriate switching can also powermotor windings where current flow therein is unidirectional.

The Control Circuitry

As stated above, the control circuitry 33 in FIG. 1 comprises fiveseparate interconnected subsystems, shown in FIG. 4, including theplugging circuit 9, the zero crossing detector 16, the commutationcircuit 17, the pulse centering circuit 25, and the voltage amplitudecontrol circuit 26. Briefly, the zero crossing detector 16 responds toinduced voltage in the stator windings to produce pulses at its outputindicating when the induced voltage in a given stator winding hascrossed through the zero voltage level. These zero crossing indicationsoccur, as will become clear later, at the time when the rotor and thestator have a precise physical relationship with respect to each other.The zero crossing indications are normally utilized by the commutationcircuit 17 to generate pulses on the output wire 86 which forms thesecond input to the OR gate 42. The signals on this wire 86 areoperative to increment the electronic stepping switch 14 in the mannerindicated above for pulses received on the wire 41 from the startcircuit 40. These pulses received on the output wire 86 also reset thestart circuit 40. Therefore, once pulses are present on the output wire86, each of these pulses will reset the start circuit 40 preventinggeneration of start pulses and will also be operative to step theelectronic stepping switch 14 each time a pulse is produced on the wire86.

The commutation circuit 17 also develops control signals that aretransmitted to the pulse centering circuit 25. The pulse centeringcircuit 25 responds to the control signals from the commutation circuit17 to generate, at the proper time, the inhibit signal, previouslymentioned, which is transmitted over the inhibit wire 56 to prevent theAND gates 52, 53, 54, and 55 from generating an active signal at any oftheir respective outputs. As will become more clear later, this inhibitsignal on the inhibit wire 56 is operative to center the power pulsesapplied to the stator windings at a time when the rotor has apredetermined position. For maximum efficiency, the power pulses arecentered about the time when the torque angle is 90°.

The voltage amplitude control circuit 26 is responsive to the zerocrossing detector 16, the commutation circuit 17, and the pulsecentering circuit 25 to generate pulses which are transmitted to themotor power supply 13 over the Up and Down wires 87 and 88,respectively. The pulses generated by the voltage amplitude controlcircuit 26 are operative to increase or decrease the voltage of pulsesapplied by the motor power supply 13 to the stator windings.

The plugging circuit 9 senses an overspeed condition and, in the eventthat an overspeed condition exists, causes application of a voltage to astator winding to retard rotation of the rotor. The plugging circuit 9responds to zero crossing indications from the zero crossing detector 16and the position of the electronic stepping switch 14 to sense prematureoccurrence of a zero crossing in which case the plugging circuit 9controls the motor power supply 13 so that a voltage is applied to astator winding to reduce the rotor speed and thereby correct theoverspeed condition. The situations in which the plugging circuit 9 isoperative and the structure and operation of the plugging circuit 9 willbe described in detail later. First, the other elements of the controlcircuit 33 in FIG. 1 will be more fully described.

Zero Crossing Detector

The A coil output of the motor power supply 13 is connected at point 89to the A coil 31 which is one stator winding for a machine of the typegenerally shown in FIG. 1. The B coil output of the motor power supply13 is connected at 90 to the B coil 32.

The zero crossing detector 16 also connects to the connection points 89and 90 so that the voltages induced in the A coil 31 and the B coil 32can be detected and a zero crossing signal generated at the outputthereof indicating the detection of an induced voltage zero crossing.Detection of a zero crossing for the induced voltage in an unpoweredstator winding, however, is not a simple matter because, when a statorwinding is switched from a conducting or powered to a non-conducting orunpowered state, the dissipation of the stored energy in the magneticfield causes two zero crossings of the voltage appearing across thewinding. Consequently, the zero crossing detector 16 must be capable ofdifferentiating between a zero crossing of the induced voltage and zerocrossings associated with the dissipation of stored energy in a statorwinding when it is switched from a powered to an unpowered state.

Referring briefly to FIG. 2, the rectangular areas labeled 34, 35, 36,and 37 represent time periods during which power pulses are applied toeither the A coil or the B coil. During the period of time between thoseperiods when a power pulse is applied, the movement of the rotor isoperative to induce a voltage in the unpowered stator windings. Forexample, for the time period between the rectangular areas 34 and 36 aninduced voltage appears across the A coil 31 which is indicatedgenerally by the dashed line 91. A zero crossing for this inducedvoltage occurs at the point when the rotor is at its 180° position. Whenthis condition is detected by the zero crossing detector 16, a zerocrossing signal is transmitted to the commutation circuit 17 indicatingthat a zero crossing for the induced voltage in the A coil 31 has beendetected. When the rotor reaches its 0° or 360° position, the zerocrossing detector 16 also transmits a zero crossing signal to thecommutation circuit 17 indicating that this zero crossing for theinduced voltage in the A coil 31 has been detected.

The zero crossing detector 16 is connected to the B coil 32, and thisdetector 16 generates a zero crossing signal at its output at times whenthe rotor is at its 90° and 270° positions which correspond to the timeswhen the induced voltage in the B coil 32 crosses through zero.

As indicated generally above, the problem of detecting zero crossingsfor the induced voltage in stator windings is a difficult one because ofthe zero crossings of the voltage appearing across a stator windingcaused by dissipation of stored field energy. A wave form showing thevoltage across a stator winding as it changes from a powered to anunpowered state is shown in FIG. 6. When a given stator winding ispowered by a signal having a positive voltage of +V, energy is stored inthe field generated thereby. When the power is removed from the statorwinding, as indicated at 94, the voltage appearing across the previouslypowered stator winding will fall very rapidly passing through zero, asshown at 95, and then develops a large negative voltage thereacross. Asshown in FIG. 4, however, zener diodes Z and diodes D are connectedbetween the points 89 and 90 and the supply voltage points +V and -V.These diodes D and zener diodes Z arrangement is operative to limit themaximum reverse voltage appearing across a previously powered statorwinding to thereby prevent damage to connected circuits. Specifically,the breakdown voltage for the zener diodes Z in the preferred embodimentis equal to three times the maximum amplitude of the winding supplyvoltage +V. As such, the maximum negative voltage developed across astator winding, as indicated in FIG. 6, is equal to -4 V as indicatedgenerally at 96. When stored energy is being dissipated, the voltageacross the stator winding will remain at -4 V until substantially all ofthe stored energy has been dissipated. Then, the voltage across thestator winding increases very rapidly and passes through zero a secondtime as shown at 97 and reaches a maximum shown at 98 at which time thevoltage across the stator winding is equal to the induced voltage inthat winding. Then, as the rotor continues to rotate, the inducedvoltage appears across the unpowered stator winding. The zero crossingdetector 16 must respond to the zero crossing of the induced voltageshown generally at 99 to produce a zero crossing signal at its outputoccurring at a time when the torque angle is 90° for the poweredwinding. Each zero crossing signal is used by the commutation circuit 17to produce pulses at the time when power should normally be switchedfrom one stator winding to another.

Referring again to FIG. 4, the zero crossing detector within the dottedline 16 includes a zero crossing detector connected to each of thestator windings shown generally at the A coil 31 and the B coil 32. Thezero crossing detector for the A coil is operational to detect the zerocrossing of the induced voltage which appears at point 89. In a similarmanner, the zero crossing detector for the B coil is operational todetect the zero crossing of the induced voltage appearing across the Bcoil at point 90.

The zero crossing detector for the A coil includes a pair of idealdifferential amplifiers 101 and 102 with the minus input of theamplifier 101 and the plus input of the amplifier 102 being electricallyconnected to the point 89. The positive input of the differentialamplifier 101 is connected to the +V output from the power supply 13while the negative input of the differential amplifier 102 is connectedto the -V output of the power supply 13. For forward rotation, thedifferential amplifier 101 is under normal conditions strobed via the ORgate 200 whenever a signal is present on the wire 61 which indicatesthat a negative power pulse is applied to the B coil. The differentialamplifier 102 is under normal conditions strobed via the OR gate 201whenever a signal is present on the wire 59 which indicates that apositive power pulse is applied to the B coil. As such, whenever thewire 61 is positive, the differential amplifier 101 will produce apositive voltage at its output if the voltage across the A coil is lessthan +V volts. Similarly, the differential amplifier 102 is operativewhenever the wire 59 is positive to produce a positive output voltagewhenever the voltage across the A coil is greater than -V volts.

The output of each of the differential amplifiers 101 and 102 isconnected to a wire 103. Depending on which wire 59 or 61 is positive,the voltage on the wire 103 will be positive when the voltage across theA coil 31 is greater than -V volts or less than +V volts respectively.As will be recalled from the discussion of FIG. 6, the voltage acrossthe A coil falls within the range where the differential amplifier 101or 102 will produce a positive output during the period of time when theA coil is changing from the powered to the unpowered state but thisperiod of time is short. As such, a delay circuit can eliminate theeffect of the zero crossings associated with power switching of the Acoil voltage. Such a delay circuit is shown within the dotted line 104.This circuit produces a positive voltage at its output if a positivevoltage is present on the wire 103 for a predetermined period of time.One way of implementing such a function is to provide a delay line 105whose input is connected to the wire 103 and whose output is connectedto one input of an AND gate 106. A second input to this AND gate 106 isconnected directly to the wire 103 so that the output of the AND gate106 will be positive only when both inputs thereto are positive. Thedelay circuit 105 is operative to delay a positive voltage fromappearing at its output for 70 microseconds after the time that theinput goes positive. As such, the output of the AND gate 106 will bepositive only if a positive voltage is present on the line 103 for aperiod of time exceeding 70 microseconds.

While the foregoing discussion has been directed to one implementationfor the circuit shown within the dotted line 104, a second approach forimplementing this function is shown within the dotted line 107 in FIG.7. It will be recognized by those skilled in the art that the circuitwithin the dotted line 107 can be implemented by numerous other possiblecircuits which perform the same function.

While the circuit within the dotted line 104 in FIG. 4 has beendescribed as producing a positive voltage at its output if a positivevoltage is on the wire 103 for a period exceeding 70 microseconds, theexact delay period required before the output goes positive differs frommachine to machine, and the value selected depends on the rise and falltime associated with the dissipation of stored energy in a statorwinding which has been switched from a powered to an unpowered state.Specifically, referring to FIG. 6, the circuit within the dotted line104 should not generate a positive voltage at the output during theperiod of time in which the voltage across a stator winding passesthrough a region such as that indicated at 108. If a positive voltagewere to appear at the output of the circuit 104 during this period oftime, a false zero crossing would be indicated. Hence, for the circuitshown in FIG. 4, the delay line 105 must have a delay longer than thetime required for the voltage across the winding to fall from thevoltage at 94 to less than -V volts. For other alternative circuits, asimilar delay must be produced to avoid false zero crossing indicationscaused by the dissipation of stored energy.

Referring again to FIG. 4, the output of the circuit 104 is connected toone input of the AND gates 111 and 112. As indicated earlier, thevoltage on this line will be positive whenever the input signalappearing on wire 103 has been positive itself for a period of timegreater than a predetermined period of time which, for the circuitshown, is the length of the delay 105. The second input to the AND gate111 is connected to the output of the OR gate 200 while the second inputof the AND gate 112 is connected to the output of the OR gate 201. Forforward rotation, the output of the OR gate 201 is positive when apositive pulse is being applied to the B coil and the output of the ORgate 200 is positive when a negative pulse is being applied to the Bcoil. The remaining input to the AND gate 111 is connected to the outputof an inverter 113. The output of this inverter 113 will be positivewhenever the induced voltage across the A coil is also positive. Theremaining input to the AND gate 112, on the other hand, is electricallyconnected to the output of an inverter 114 whose output will be positivewhenever the voltage appearing across the A coil is negative.Consequently, the output of the AND gate 111 will be positive when theinduced voltage across the A coil is positive, the output of the OR gate200 is positive indicating that a negative power pulse is being appliedto the B coil, and the voltage induced across the A coil has been lessthan +V volts for a time period greater in time than that of the delay105. This condition occurs just after the induced voltage across the Acoil goes through zero volts going towards a positive voltage.Similarly, the output of the AND gate 112 is positive whenever thevoltage induced in the A coil is negative, the output of the OR gate 201is positive indicating that a positive power pulse is being applied tothe B coil, and the induced voltage in the A coil has been greaterthan - V volts for a period of time greater than the time of the delay105. This condition occurs just after the induced voltage across the Acoil goes through zero volts going negative.

When the output of either AND gate 111 or 112 is positive, the OR gate115 output becomes positive and comprises a zero crossing signal on wire116 from the zero crossing detector 16. This output wire 116 isconnected to the set input of a latch 117 and the reset input of a latch118 in the commutation circuit 17.

The zero crossing detector for the B coil is essentially identicalcircuitwise to the zero crossing detector for the A coil. The zerocrossing detector for the B coil includes two differential amplifiers120 and 121. The strobe inputs for the differential amplifiers 120 and121 are connected via the OR gates 202 and 203 to the wires 58 and 60,respectively, for forward rotation of the motor.

The output of the zero crossing detector for the B coil appears on theoutput wire 122. The wire 122 is connected to the reset input of thelatch 117 and the set input of the latch 118 of the commutation circuit17.

For reverse rotation, the strobe inputs for the differential amplifiers101 and 102 are reversed and the strobe inputs for the differentialamplifiers 120 and 121 are reversed. That is, the strobe input for thedifferential amplifier 101 is connected via the OR gate 200 to the wire59 while the strobe input for the differential amplifier 102 isconnected via the OR gate 201 to the wire 61. Also, the strobe input forthe differential amplifier 120 is connected via the OR gate 202 to thewire 60 while the strobe input for the differential amplifier 121 isconnected via the OR gate 203 to the wire 58. This reverse connectioncan be achieved very simply by a four pole/double throw switch (notshown) connected to accomplish this function.

In summary, each zero crossing detector produces a pulse at its outputwhich is herein referred to as a position pulse. The output positionpulse is produced by each zero crossing detector at a time when therotor field is aligned with the magnetic axis of the stator winding onwhich the induced voltage crosses through zero. In the machine describedabove with two stator windings, the position pulse is generated when thetorque angle is 90° for the powered stator winding. As such, a positionpulse is generated four times for each rotor revolution for the twostator winding machine shown at times when the position of the rotorwith respect to the stator is known and corresponds to the midpoint orsymmetry point for the power pulse applied at the time of the positionpulse generation.

It will be recognized by those skilled in the art that the back emf atvoltages other than zero volts may be used also to determine rotorposition. One such other approach is to detect when the back emf on twowindings is the same amplitude which indicates a discrete rotorposition, as, for example, 135° in a two phase machine.

While the above discussion provides a sensor in a two phase/two polemachine for detecting rotor position four times each rotor revolution,the position sensor may be modified to sense rotor position at othertime intervals such as once per rotation. Such a modification, however,would require other modifications to be made in the remainder of thesystem.

The above described circuitry may also be used in machines having morethan two poles without modification.

Commutation Circuit

As indicated generally above, the commutation circuit 17 generatespulses at its output for controlling the electronic stepping switch 14which in turn determines the normal sequence of application of voltagesto maintain rotor rotation. The commutation circuit 17 includes thelatches 117 and 118. The latch 117 becomes set, and the latch 118becomes reset, whenever a zero crossing is detected for the inducedvoltage in the A coil. Similarly, the latch 118 becomes set, and thelatch 117 becomes reset, whenever a zero crossing is detected for theinduced voltage in the B coil. As will become more clear later, thesetting and resetting of the latches 117 and 118 is operative toactivate the up/down counters 18 and 19 in the commutation circuit 17 ina manner to be described in greater detail.

The commutation circuit 17 includes a square wave generator 123 whichproduces square wave pulses at its output having a frequency of f. Whilethe frequency f is not critical, in the preferred embodiment of theinvention the square wave pulses are generated at a rate of at least50,000 pulses per second. The output pulses of the square wave generator123 are utilized directly by the commutation circuit 17, the pulsecentering circuit 25, and the voltage amplitude control circuit 26. Theoutput of the square wave generator 123 is also connected to a flip-flop124 which produces at its output a square wave having a frequency off/2. This square wave signal at the frequency of f/2 is also utilizeddirectly by the commutation circuit 17.

The output of the latch 117 is connected to one input of an AND gate 125and to one input of a second AND gate 126. The second input to the ANDgate 125 is connected to the output of the flip-flop 124 so that theoutput of the AND gate 125 will be a series of pulses at a frequency off/2 whenever the latch 117 is set. The other input to the AND gate 126is connected to the output of the square wave generator 123 so that theoutput of the AND gate 126 is a series of pulses at a frequency of fwhenever the latch 117 is set.

The output of the latch 118 is connected to one input of an AND gate 127and also to one input of a further AND gate 128. The second input to theAND gate 127 is connected to the output of the square wave generator 123so that the output of the AND gate 127 comprises a series of pulses at afrequency of f whenever the latch 118 is set. The second input to theAND gate 128 is connected to the output of flip-flop 124 so that the ANDgate 128 output comprises a series of pulses at a frequency of f/2whenever the latch 118 is set.

The commutation circuit 17 includes the two up/down counters 18 and 19.Each of the up/down counters 18 and 19 includes an up input and a downinput, the counters being operative to count up when pulses are appliedto the up input and operative to count down when pulses are applied tothe down input. In addition, up/down counters 18 and 19 have borrowoutputs 130 and 131, respectively, each of which produces a borrow pulsewhenever the respective up/down counter 18 or 19 counts through zero andgoes negative. Each output 130 and 131 is connected to one input of anOR gate 132 which passes a borrow pulse on either input line to itsoutput. The output of the OR gate 132 is connected to the set input ofthe logic latch 23. The significance of the logic latch 23 will becomemore clear later.

Each up/down counter 18 or 19 has control circuitry associated therewithwhich prevents the counter itself from ever counting in the negativedirection. Consequently, whenever either of these counters 18 or 19begins to count up because it receives pulses at its up input, thecounter will always begin counting upwardly from a value of zero.

The up input to the up/down counter 18 is connected to the output of theAND gate 125 while the down input is connected to the output of the ANDgate 127. The counter 18, consequently, will count up at a rate of f/2whenever the latch 117 is set. On the other hand, the counter 18 willcount down at a rate of f whenever the latch 118 is set. On the otherhand, the counter 19 has its up input connected to the output of the ANDgate 128 and the down input connected to the output of the AND gate 126.Consequently, the counter 19 will count up at a rate of f/2 whenever thelatch 118 is set and will count down at a rate of f whenever the latch117 is set.

The significance of the up/down counters 18 and 19 is that each counter,when it is counting up, will count up for a period of time whichcorresponds to the time actually taken for the rotor of the machine torotate 90°. On the other hand, since the up/down counters 18 and 19,when they count down, count down at a frequency twice as fast as theycount up, the borrow pulse or second position pulse generated by eitherup/down counter 18 or 19 is generated at a time which corresponds to thetime when the machine rotor has rotated 45° beyond the point when a zerocrossing of the induced voltage in one stator winding has been detected.As such whenever a borrow pulse is generated, this borrow pulse normallyindicates that the rotor is positioned so that power should be switchedfrom one stator winding to another in accordance with the sequenceestablished by the electronic stepping switch 14 if each stator windingis powered for a period of time equal to the time required for themachine rotor to rotate 90°. If pulse widening or shortening isoccurring, the switching of power pulses occurs at some other time aswill become clearer later.

A timing chart showing the periods of time when the up/down counters 18and 19 are counting up and down is shown in the lower half of FIG. 2.The times when these counters 18 and 19 are counting up and down areshown in relationship to the power pulses and the induced voltages inthe A coil 31 and the B coil 32.

The commutation circuit 17 also includes the rate variable oscillator 21which produces pulses at its output at a selected rate. The outputpulses from the rate variable oscillator 21 are transmitted over a wire133 to the set input of the speed latch circuit 22. The rate variableoscillator 21 produces pulses at a selectable rate, however, thefrequency for the machine shown in FIG. 1 for these pulses is such thatfour spaced pulses are generated during the period of time in which therotor makes one complete revolution at the selected speed of rotation.

The output of the logic latch 23 is connected via a wire 134 to oneinput of an AND gate 24. The second input to this AND gate 24 isconnected to the output of the speed latch 22. When both the logic latch23 and the speed latch 22 are set, the proper input conditions to theAND gate 24 are met so that a positive voltage will appear at the output135. This output is connected via the wire 86 to one input to the ORgate 42. Consequently, when positive voltage appears at the output ofAND gate 24, the electronic stepping switch 14 is stepped to therebynormally cause a different stator winding to be actuated by the motorpower supply 13.

Since both the logic latch 23 and the speed latch 22 must be set beforea positive voltage is generated on the wire 86 for stepping theelectronic stepping switch 14, the logic latch 23 may indeed be set at atime prior to the setting of the speed latch 22 or vice versa. The logiclatch 23 is normally set before the speed latch 22, a condition whichoccurs when the machine is operating synchronously at the speed selectedby the rate variable oscillator 21. Consequently, when a switching pointis indicated by the setting of the logic latch 23, the actual switchingof the stepping switch is delayed until the speed latch 22 is set. Assuch, the leading edge of a power pulse applied to a stator winding isdelayed by a period of time corresponding to the delay between thesetting of the logic latch 23 and the setting of the speed latch 22.This switching delay varies depending on the load torque on the machineand will be discussed in greater detail in connection with the pulsecentering circuit 25.

When the speed latch 22 becomes set prior to the setting of the logiclatch 23, the machine speed is below that of the selected speed ascontrolled by the rate variable oscillator 21. As such, as soon as therotor position reaches the proper physical position for switching powerfrom one stator winding to another, the logic latch 23 will be set in amanner previously described, and the electronic stepping switch 14 willadvance to change the stator winding to which a power pulse is applied.

Both the logic latch 23 and the speed latch 22 have their reset inputsconnected together and each of these are connected to the output of adelay 136. The input of this delay 136 is connected to the output of theAND gate 24 so that whenever both the logic latch 23 and the speed latch22 are set, they will be reset automatically at a period of time equalto the delay time of the delay circuit 136 after the output of the ANDgate 24 goes positive. As such, when both latches 23 and 22 become set,they will automatically be reset at a predetermined period of timelater. Preferably, the delay 136 is 20 microseconds although a longer orshorter delay time can be utilized depending on the speed of the othercircuits connected to the AND gate 24 output.

While a delay line can be used for the delay 136, other circuits such asa pulse stretcher which introduces a delay between the receipt of aninput signal and the generation of its output signal may also beutilized. It will also be clear to those of skill in the art that otherforms of circuitry may be utilized for resetting the logic latch 23 andthe speed latch 22 so long as the pulse which appears at the output ofthe AND gate 24 has a sufficient duration to step the electronicstepping switch 14.

In summary, the zero crossing detector for detecting a zero crossing ofthe induced voltage appearing in the A coil 31 produces a pulse whichcauses the up/down counter 18 to count up at a rate of f/2. The counter18 will continue to count up at a rate of f/2 until the zero crossingdetector connected to the B coil 32 detects a zero crossing of theinduced voltage in the B coil. When this latter zero crossing isdetected, the number stored in the counter 18 is representative of thetime required for the rotor to rotate 90°. Also, when the zero crossingis detected on the B coil, the rotor is positioned so that the torqueangle between the rotor field and the stator field produced by the Acoil is exactly 90°. Thereafter, the rotor must rotate no more than 45°before power is normally switched from the A coil 31 to the B coil 32.

When a zero crossing of the induced voltage is detected across the Bcoil 32, the counter 18 immediately begins to count down at a rate of fand the counter will generate a borrow pulse or second position pulse atits output when the counter reaches zero. Since the counter 18 iscounted down at a rate twice as fast as it was counted up, the counter18 will reach a value of zero in a period of time after it startscounting down which corresponds to the time required for the rotor torotate 45°. As such, when the borrow pulse or second position pulse isgenerated at the output of the counter 18, the rotor is positionedexactly at a point where the torque angle is 45° and power shouldnormally be switched from the A coil to the B coil.

The counter 19 operates in a manner similar to the counter 18 to produceborrow pulses or second position pulses at its output at timescorresponding to the time when power should normally be switched fromthe B coil 32 to the A coil 31. That is, the pulse from the counter 19is generated at a time when the torque angle is 45°.

Pulse Centering Circuit

The pulse centering circuit 25 is operative to insure that the powerpulses applied to a given stator winding are applied at times alwayssymmetric about the time when the torque angle is a predetermined value.That is, for a machine having a desired average torque angle of, forexample, 90°, the power pulses applied to stator windings must becentered at the time when the torque angle is 90°. For less than maximummachine efficiency, the power pulses can be applied to stator windingswhen the torque angle is other than 90°.

The pulse centering circuitry 25 centers the power pulses applied tostator windings in the following manner. In general, the pulse centeringcircuit detects the time period between the 45° firing point and theactual pulse firing and foreshortens the pulse at its trailing edge bythat same period. More specifically, when the logic latch 23 becomesset, a condition which occurs when the rotor is at a 45° position, apositive voltage indicating that the logic latch is set is transmittedover a wire 137 to the start input of a counter 138. This signal isoperative to turn on the counter 138 which counts pulses on the wire 149from the square wave generator 123 at a rate of f. The counter 138counts up at a rate of f until the speed latch 22 is set. When the speedlatch 22 is set, a positive voltage is transmitted over a wire 139 tothe stop input of the counter 138, causing the counter 138 to stopcounting. The count stored in the counter 138, after being stopped, is ameasure of the time between the setting of the logic latch 23 and thesetting of the speed latch 22. As such, the counter 138 contains a countindicative of the time that the power pulse application has been delayedbecause the logic latch 23 was set prior to the speed latch 22. In otherwords, the counter 138 stores a number indicating the extent of leadingedge pulse shortening which has occurred as a result of the operation ofthe commutation circuit 17.

The information stored in the counter 138 is utilized to shorten thetrailing edge of the power pulse applied to a stator winding. This pulseshortening of the trailing edge is accomplished by a comparison circuit140. The value of the counter 138 is transmitted to the comparisoncircuitry 140 via cables shown at 142. The counter 138 data is comparedin comparison circuit 140 with the value of either counter 18 or 19,depending on which counter 18 or 19 is being counted down. Each bitposition of the counters 18 and 19 is connected to one input of one ANDgate such as 143 and 144 respectively. The second input to the AND gate143 is connected to the output of the latch 118 which indicates that thecounter 18 is counting down. The second input to the AND gate 144 isconnected to the output of the latch 117 which indicates that thecounter 19 is counting down. The output of each AND gate 143 and 144 isconnected to one input of an OR gate 145 whose output is then connectedto the input to the compare circuit 140.

The compare circuit 140 is operative to determine when the stored valuein either counter 18 or 19 counts down to the value stored in thecounter 138. That is, whenever the counter 18 or 19, whichever counteris counting down, reaches a value identical to that stored in thecounter 138, the compare circuit 140 generates an inhibit signal at itsoutput which is connected via the wire 56 to one input of each of theAND gates 52, 53, 54 and 55. The inhibit pulse on the inhibit wire 56will remain active until a pulse is generated on the wire 86 indicatingthat a power pulse should be applied to the next stator winding insequence as controlled by the electronic stepping switch 14. Therefore,the operation of the pulse centering circuit 25 is to generate aninhibit signal on the inhibit wire 56 at precisely the time required sothat the power pulse applied to a given stator winding is symmetricabout the time when the torque angle, for the powered stator winding, ison average equal to a preselected value which, for the circuitdiscussed, the pulses are symmetric about the time when the torque angleis 90°.

In summary, the pulse centering circuit 25 and the commutation circuit17 operate together to widen or narrow the power pulses applied to thestator windings. The power pulses are widened when the load torqueincreases and narrowed when the load torque decreases. The speed of themachine, however, remains at the selected speed as defined by thesetting of the rate variable oscillator 21 and the average torque angleremains constant.

Motor Power Supply Voltage Controls

The voltage output of the motor power supply 13 shown in FIG. 4 may becontrolled by the power supply voltage amplitude control circuit 26. Thevoltage amplitude control circuit 26 produces signals at either its upor down output for either increasing or decreasing the pulse amplitudeproduced at the power supply 13 output. As already indicated, byincreasing or decreasing the amplitude of power pulses applied to thestator windings, the machine can run at a constant speed and at apredetermined efficiency even when the load torque varies widely, andthe width of the power pulses can be held within a predetermined range.

The voltage control circuit 26 includes a modulo N counter 148, acounter which produces a single pulse at its output each time N pulsesare received at its input. The value stored in the counter 138 istransmitted over the multi-wire cable 142 to the modulo N counter 148.This stored value in the counter 138 comprises the N input into themodulo N counter 148. A second input to this counter 148 comprises thesquare wave signal on line 149 from the square wave generator 123. Themodulo N counter 148, however, is operative only when an enable signalis present at the enable input thereto. The enable signal is presentfrom the time when a stator winding is switched to a powered state,i.e., current is conducting therethrough, until the next zero crossingis detected by a zero crossing detector. The enable signal is generatedby an enable latch 150. This enable latch 150 is set whenever a pulseappears on the wire 86 which occurs when the commutation circuit 17determines that power should be switched from one stator winding toanother. The enable latch 150 is reset whenever the next zero crossingis detected by the zero crossing detector 16. The wires 116 and 122comprise inputs to an OR gate 151 whose output is connected to the resetinput of the enable latch 150. Thus, the latch 150 is reset when a zerocrossing is detected as indicated by a positive voltage on either wire116 or 122.

The modulo N counter 148 when enabled, as indicated above, produces atits output 152 a pulse for every N pulses received from the square wavegenerator 123. Each output pulse on the output wire 152 is counted by acounter 153 shown in FIG. 4 as the K5 counter. As such, the counter 153holds a number after the counter 148 turns off which is an indication ofthe pulse width of power pulses applied to stator windings. For example,if the value stored in the counter 153 is large, this indicates that thecount stored by the counter 138 is low and, therefore, the turnon delay,i.e., the time delay between the setting of the logic latch 23 and thesetting of the speed latch 22, is small. As such, each power pulseapplied to a stator winding is long in time and is approaching the timeperiod required for the rotor to turn 90°. Under this condition, themachine is operating with relatively high load torque, and the powerpulses are wide to prevent slow down.

On the other hand, should the value of the counter 153 be small, thisindicates that the count stored by the counter 138 is very large andthat the power pulses applied to stator windings are relatively short.Under this condition, the machine is operating with a relatively lowload torque and the power pulses are short to prevent speed up.

As indicated above, the objective of the voltage control circuit 26 isto incrementally vary the amplitude of pulses applied to stator windingsso as to respond to large load torque variations in a manner whichmaintains constant speed at a selectable torque angle. Power pulseamplitude changing also permits the machine to operate with power pulseshaving a duration close to the time required for the rotor to rotate 90°at the selected speed thereby evening the generated torque during eachrevolution of the rotor. To accomplish this objective, the value storedin the counter 153 at the conclusion of the counting period, i.e., whenthe enable latch 150 is reset, is compared with a preset low value by acomparison circuit 154. If the value stored in the counter 153 is lessthan a predetermined value set into the comparison circuit 154 bymechanical switches or the like as indicated generally by the input line155, then the comparison circuit 154 will generate a signal at itsoutput 156 indicating the value of the counter 153 is less than thepredetermined value. This indicates that the width of the power pulseapplied to a stator winding is less than a predetermined desired value.

The count stored in the counter 153 is also transmitted over a wire 158to a second comparison circuit 159. This second comparison circuit 159receives from preset switches or the like, as indicated generally by theinput line 160, a number from switches (not shown) indicating the highor upper value acceptable for the number which is stored in the counter153 during the period while the enable latch 150 is set. If the actualcount stored in counter 153 during the enable period is greater than theacceptable upper value, then the comparison circuit 159 generates apulse at its output 161 indicating that the value of counter 153 hasexceeded the acceptable upper value. This condition indicates that thepulse width of the power signal applied to a stator winding is greaterthan desired.

When the output of the comparison circuit 154 indicates that the counter153 is less than a preset lower limit, this condition indicates that thepower applied to a given stator winding should be reduced. As such, thissignal could be utilized directly by the motor power supply to decreasethe voltage applied to each of the stator windings. However, in order toavoid undue "hunting" by the motor power supply 13, i.e., unnecessaryvoltage changing, a comparison counter 162 is provided to count thenumber of pulses, which indicate a voltage decrease request, from thecomparison circuit 154. If the comparison counter 162 receives apredetermined number of voltage decrease request pulses, as set on apreset line 163 by switches or the like (not shown), followingsuccessive periods of time during which the enable latch 150 is set,then the comparison counter 162 will generate a pulse on its output 88indicating that the voltage of the motor power supply should be reduced.This pulse is connected to the down input of the motor power supply 13which is operative in the manner previously described to incrementallydecrease the voltage of power pulses applied to the machine statorwindings.

When the comparison circuit 159 generates a pulse at its output 161,this is an indication that the value stored in the counter 153 duringthe period when the enable latch 150 was set exceeds a predeterminedhigh value as set by preset switches or the like indicated generally bythe input line 160. The pulse generated at the output 161 indicates thatthe power pulses being applied to the stator windings are wide and thatthey exceed the desired predetermined maximum width. Consequently, theamplitude of power pulses applied to stator windings should beincreased. Again, in order to avoid undo hunting by the motor powersupply 13, a comparison counter 164 is provided to count successiveamplitude increase requests received on the line 161, and, if the numberof these successive requests, i.e., requests generated by the comparisoncircuit 159 following successive periods of time when the enable latch150 is set, exceeds the preset amount indicated by the preset numberinput 163, then the comparison counter 164 produces a pulse on the wire87 which is connected to the up input to the motor power supply 13. Thispulse is operative in a manner described earlier to incrementallyincrease the amplitude of power pulses applied to stator windings by themotor power supply 13.

As indicated above, the comparison counter 154 produces a pulse at itsoutput wire 156 if the value stored in the counter 153 is less than thepreset value input 155. The comparison counter 154 produces a pulse onan output wire 167 if the value stored by the counter 153 is equal to orgreater than the preset value input 155. The pulses on the wire 167 areutilized to reset the comparison counter 162 to zero. Therefore, thecomparison counter 162 will generate a pulse at its output 88 only if apreset consecutive number of pulses, as defined by the preset numberinput 163, occur on the wire 156. In this manner, a delay is built intothe system before the power pulse amplitude is changed to assure thatshort duration circuit transients do not trigger the amplitude changecircuitry.

In a similar manner, the comparison counter 159 produces a pulse at itsoutput wire 161 if the value stored in the counter 153 is greater thanthe preset upper value input at the input 160. If the counter 153 isless than or equal to the upper value 160, then the counter 159 producesa pulse on the wire 168 which is connected to the reset of thecomparison counter 164. Consequently, requests to increase the powerpulse amplitude on the wire 87 are not generated unless a predeterminednumber, as defined at 163, of such consecutive requests are generated onthe wire 161.

In summary, the voltage control circuit 26 is operative to increase ordecrease the power pulse amplitude in response to large increases ordecreases in load torque or changes in line voltage so as to maintainoperation at a selectable speed and at a selected average torque angle.

Plugging Control Circuit

The plugging circuit 9 remedies overspeed conditions which might developduring operation of the motor 10 in FIG. 1. The plugging circuit 9assures synchronous operation of the motor 10 at the constant selectablespeed established by the rate variable oscillator 21. The condition ofthe motor 10 which leads to the functioning of the plugging circuit 9will first be described so as to facilitate an understanding of thestructure and operation of the plugging circuit 9.

With reference to FIG. 17, the voltage across the stator windings isplotted as a function of time. The curve 251 represents the voltageacross the A coil, and the curve 252 represents the voltage across the Bcoil. The time is divided into periods, 1, 2, 3, 4, 1', 2', 3', 4', 1",2", 3", etc.

Time period 1 corresponds generally to a portion of the operationalsequence in FIG. 2 and depicts normal operation of the motor 10.Specifically, a positive voltage pulse 253 in FIG. 17 corresponds to thevoltage pulse 34 in FIG. 2. The voltage pulse 253 is applied at thebeginning of time period 1 and is removed at approximately the end oftime period 1. The voltage pulse 253 is applied symmetrically about thetime at which the voltage induced across the B coil crosses through zerovolts as indicated at 254. Consequently, the motor 10 is operating atconstant selectable speed and constant average torque angle with thevoltage pulse width and symmetry being maintained by the pulse centeringcircuit 25 and the voltage pulse amplitude being adjusted by the voltageamplitude control circuit 26 to keep the voltage pulse width withinpredetermined limits. Normal operation continues as time period 2commences. Thus, a positive voltage pulse 255 in FIG. 17, whichcorresponds to voltage pulse 35 in FIG. 2, is applied to the B coil atthe beginning of time period 2. The voltage pulse 255 is appliedsymmetrically about the time at which the voltage induced across the Acoil crosses through zero volts as indicated at 256.

The above relates to normal operation. Suppose that an event occurswhich tends to increase the rotor speed. As shown in FIG. 17, a sharprise in the line voltage producing the spike 257 may be the event thattends to increase the rotor speed. The rotor speed may increase,however, due to any one of a number of events well-known to those ofskill in the art, for example, removal of the load from the motor 10.Manual decrease of the speed established by the rate variable oscillator21 during normal operation also produces an overspeed condition.

An overspeed condition may also develop during startup of the motor 10.Initially, the start circuit 40 in FIG. 4 controls the electronicstepping switch 14 to cause the motor power supply 13 to apply theproper voltage in the correct sequence to the stator windings. When,however, the rotor speed is such that the rotating magnetic field of therotor induces a detectable voltage in the stator windings, thecommutation circuit 17 assumes control of the electronic stepping switch14. When the commutation circuit 17 becomes operative, the pulsecentering circuit 25 and voltage amplitude control circuit 26 alsobecome operative such that they increase the width and/or amplitude ofthe voltage pulses applied to the stator windings if the rotor is not atthe selectable speed established by the rate variable oscillator 21.Eventually, the instantaneous rotor speed will equal the selectablespeed established by the rate variable oscillator 21, but the width andamplitude of the voltage pulses may be such as to accelerate the rotorabove the desired speed creating an overspeed condition. Stateddifferently, the operation of the pulse centering circuit 25 and thevoltage amplitude control circuit 26 tend to cause the rotor speed toovershoot the selectable speed established by the rate variableoscillator 21 during start-up. A similar situation may develop duringoperation due to manual increase of the speed established by the ratevariable oscillator 21.

Furthermore, if overshoot occurs, the pulse centering circuit 25 andvoltage amplitude control circuit 26 become operative such that theydecrease the width and/or amplitude of the voltage pulses applied to thestator windings in an attempt to synchronize the speed of the rotor tothe selectable speed established by the rate variable oscillator 21.Consequently, deceleration of the rotor by frictional load,gravitational forces, etc. reduces rotor speed. The deceleration,however, may cause the rotor speed to undershoot the speed establishedby the rate variable oscillator 21. Hence, the voltage pulse widthand/or amplitude are increased. Consequently, the rotor speed may againovershoot the selectable speed. This sequence may repeat so that therotor speed oscillates about the selectable speed, thereby resulting ininstability of the motor 10. Thus, the motor 10 may not becomesynchronous at the speed established by the rate variable oscillator 21.

Referring again to FIG. 17, the rotor speed is shown to increase afterthe event 257 occurs. Since the zero crossings of the induced voltage inthe stator windings occur at certain orientations of the rotor withrespect to the stator, the rate at which the zero crossings of theinduced voltage occur is an indication of rotor speed. Hence, theincrease in rotor speed is observed in FIG. 17 by reason of the increasein the rate of occurrence of the zero crossings. This is evidenced bythe fact that the zero crossings of the induced voltage in the B coilappear earlier in each of the time periods 3, 1' and 3' whereas the zerocrossings of the induced voltage in the A coil appear earlier in each ofthe time periods 4 and 2' and, as shown, a zero crossing 258 of thevoltage induced across the A coil occurs at the beginning of time period4'.

That the rotor is overspeed is best evidenced by the fact that the zerocrossing 260 of the induced voltage across the B coil occurs prior tothe beginning of time period 1". That is, since the zero crossing 260 ofthe induced voltage across the B coil occurs prior to the beginning oftime period 1", the motor 10 is overspeed.

As shown in FIG. 17, a negative voltage is preferably applied to the Acoil so as to produce a plugging voltage 261 between the time that thezero crossing 260 of the voltage induced across the B coil occurs andthe beginning of time period 1". By plugging in the A coil, the pluggingvoltage 261 appears near the time that the torque angle between therotor and the A coil is approximately 90°. This gives near maximumplugging torque per ampere of plugging current and therefore provides anefficient method for retarding rotor speed. Of course, plugging could bedone in a different manner such as by simultaneously applying a negativevoltage to the A coil and the B coil or by applying a negative voltageto the B coil instead of the A coil from the time that the zero crossing260 of the induced voltage across the B coil occurs and the beginning oftime period 1". By plugging in the B coil, however, the plugging voltagewould appear near the time that the torque angle between the rotor andthe B coil is approximately 0°. This gives near minimum plugging torqueper ampere of plugging current and therefore is an inefficient methodfor retarding rotor speed. Nevertheless, where it is desired to usemaximum available plugging torque to slow the rotor, plugging shouldsimultaneously occur in both the A coil and the B coil.

The plugging current is a function of the difference between the appliedvoltage and the voltage which is induced across the stator winding whichis plugged. As shown in FIG. 17, the voltage which is induced across theA coil is near maximum and approximately equal and opposite to theapplied, or plugging, voltage 261. This yields a plugging current whichis quite high. Because torque is a function of current, torques due toplugging are also high, thus slowing the rotor rapidly as is indicatedin FIG. 17. Consequently, the overspeed condition is remedied almostimmediately, and the rotor speed returns to synchronism at theselectable speed established by the rate variable oscillator 21 in FIG.4.

FIG. 17 shows an example of one case in which plugging occurs. Ofcourse, an overspeed condition could occur at any point during theoperation of the motor 10. From FIG. 17, however, it can be seen thatplugging preferably commences at the time that a zero crossing occurs inthe time period preceding that in which it is supposed to occur.Plugging preferably continues until the beginning of the next timeperiod, that is, until the beginning of the time period during which thezero crossing is supposed to occur when the motor 10 is operating at setspeed.

Having described the situations in which plugging results and havingpresented an example of plugging and its effect on operation of themotor 10, an embodiment of the plugging circuit 9 will now be describedin connection with FIG. 4. Generally, in the case of the motor 10, if azero crossing occurs in the voltage induced across one stator windingduring a time period prior to the time period in which it is supposed tooccur, a voltage is applied to the second stator winding with anopposite polarity to the induced voltage appearing across the secondstator winding. This will result in the desired plugging to remedy anoverspeed condition and synchronize the rotor speed to the selectablespeed established by the rate variable oscillator 21.

In order to sense when a zero crossing of the induced voltage across astator winding occurs in a prior time period due to an overspeedcondition, the differential amplifiers of the zero crossing detector 16must be strobed at the proper times. As described earlier, the normalsequence for application of voltage to the motor 10 for rotation in theforward direction is positive voltage to the A coil, positive voltage tothe B coil, negative voltage to the A coil, negative voltage to the Bcoil, positive voltage to the A coil, etc. Also, during the time thatpositive voltage is being applied to the A coil, differential amplifier120 is strobed to enable detection of a positive going zero crossing ofthe voltage induced across the B coil. During the time that positivevoltage is being applied to the B coil, differential amplifier 102 isstrobed to enable detection of a negative going zero crossing of thevoltage induced across the A coil. During the time that negative voltageis being applied to the A coil, differential amplifier 121 is strobed toenable detection of a negative going zero crossing of the voltageinduced across the B coil. Finally, during the time that negativevoltage is being applied to the B coil, differential amplifier 101 isstrobed to enable detection of a positive going zero crossing of thevoltage induced across the A coil. As described earlier in connectionwith FIG. 17, in the case of an overspeed condition, a zero crossingwill occur during a time period preceding that in which it is supposedto occur. In FIG. 17, for example, the positive going zero crossing 260of the voltage induced across the B coil appears in time period 4'instead of time period 1". To sense an overspeed condition, therefore,differential amplifiers 101, 102, 120, and 121 in FIG. 4 must also bestrobed at times to enable detection of a zero crossing which occursearlier than it is supposed to occur.

When an inhibit signal appears on line 56, the output of inverter 204will be positive. This indicates that no voltage is being applied to theA coil or B coil such that the voltage that appears across either ofthese stator windings is voltage induced thereacross by the rotatingrotor field. Hence, zero crossings will represent zero crossings ofinduced voltage.

The output of inverter 204 is connected to one input of each of the ANDgates 205, 206, 207, and 208. The second input of the AND gate 206 isconnected to the line 47. The line 47 as indicated earlier will bepositive when the electronic stepping switch 14 indicates that apositive voltage should normally be applied to the A coil. As shown inFIG. 17, this corresponds to time periods 1, 1', 1", etc. which are thetime periods preceding those in which a negative going zero crossing ofthe voltage induced across the A coil normally occurs. Therefore, theoutput of the AND gate 206 will be positive when no voltage is beingapplied to the A coil or the B coil and during the time period precedingthat during which a negative going zero crossing of the voltage inducedacross the A coil is supposed to occur.

The output of the AND gate 206 is connected to a second input of the ORgate 201. Consequently, the differential amplifier 102 will be strobedso as to enable detection of a premature negative going zero crossing ofthe voltage induced across the A coil that is indicative of an overspeedcondition.

The second inputs of the AND gates 205, 207, and 208 are connected tolines 49, 50, and 48, respectively, when rotation of the motor 10 is inthe forward direction. The outputs of the AND gates 205, 207, and 208are connected to the second inputs of the OR gates 200, 202, and 203,respectively. Thus, the differential amplifier 121 will be strobed so asto enable detection of a premature negative going zero crossing of thevoltage induced across the B coil that is indicative of an overspeedcondition. In a similar manner, the differential amplifier 101 will bestrobed so as to enable detection of a premature positive going zerocrossing of the voltage induced across the A coil evidencing anoverspeed condition. Furthermore, the differential amplifier 120 will bestrobed so as to enable detection of a premature positive going zerocrossing of the voltage induced across the B coil which indicates anoverspeed condition as shown at 260 in FIG. 17.

The plugging circuit 9 controls the motor power supply 13 to applyvoltage so as to plug the appropriate stator winding when an overspeedcondition is indicated. As described in connection with FIG. 17, thevoltage caused to be applied by plugging circuit 9 has an oppositepolarity to the voltage induced across the stator winding which isplugged.

The strobing of the differential amplifiers 101, 102, 120, and 121 toenable detection of an overspeed condition has been described above. Theoperation of the zero crossing detector 16 has been described earlier.Therefore, when an overspeed condition is present, one of the lines209-212 will be positive. As shown in FIG. 4, lines 209-212 areconnected to one input of the AND gates 213-216, respectively.

The second input of the AND gate 214 is connected to line 47. When anegative going zero crossing of the voltage induced across the A coiloccurs during a time period preceding the one in which it normallyshould occur, both inputs to the AND gate 214 will be positive.Consequently, the output of the AND gate 214 will be positive and willset the latch 217. When the latch 217 is set, its output is positive,and, since the output of the latch 217 is connected to the second inputof the OR gate 72, the output of the OR gate 72 will be positive.Consequently, the motor power supply 13 will apply a negative voltage soas to plug the B coil.

The second input of the AND gate 216 is connected to line 48. When anegative going zero crossing of the voltage induced across the B coiloccurs during a time period preceding the one in which it normallyshould occur, both inputs to the AND gate 216 will be positive.Consequently, the output of the AND gate 216 will be positive and willset the latch 218. When the latch 218 is set, its output is positive,and, since the output of the latch 218 is connected to the second inputof the OR gate 68, the output of the OR gate 68 will be positive.Consequently, the motor power supply 13 will apply a positive voltage soas to plug the A coil.

The second input of the AND gate 213 is connected to line 49. When apositive going zero crossing of the voltage induced across the A coiloccurs during a time period preceding the one in which it normallyshould occur, both inputs to the AND gate 213 will be positive.Consequently, the output of the AND gate 213 will be positive and willset the latch 219. When the latch 219 is set, its output is positive,and, since the output of the latch 219 is connected to the second inputof the OR gate 69, the output of the OR gate 69 will be positive.Consequently, the motor power supply 13 will apply a positive voltage soas to plug the B coil.

Finally, the second input of the AND gate 215 is connected to line 50.When a positive going zero crossing of the voltage induced across the Bcoil occurs during a time period preceding the one in which it normallyshould occur, both inputs to the AND gate 215 will be positive.Consequently, the output of the AND gate 215 will be positive and willset the latch 220. When the latch 220 is set, its output is positive,and, since the output of the latch 220 is connected to the second inputof the OR gate 71, the output of the OR gate 71 will be positive.Consequently, the motor power supply 13 will apply a negative voltage soas to plug the A coil as shown in FIG. 17 at 261.

Thus, when an overspeed condition is present, the plugging circuit 9causes the motor power supply 13 to apply a voltage so as to plug one ofthe stator windings. This plugging voltage is applied at a time that anoverspeed condition is indicated by the zero crossing detector 16 due tothe occurrence of a zero crossing earlier than it is supposed to occurat the selectable speed established by the rate variable oscillator 21.

As shown in FIG. 4, the reset terminal of each of the latches 217-220 isconnected to the line 86. Consequently, when the next time period, thatis, the one in which the zero crossing would have occurred but for theoverspeed condition, commences, the plugging voltage is removed due tooperation of the commutation circuit 17.

When rotation of the motor 10 is in the reverse direction, line 47 isconnected in place of line 49 to the second input of the AND gate 205.Also, line 49 is connected in place of line 47 to the second input ofthe AND gate 206. Similarly, line 48 is connected in place of line 50 tothe second input of the AND gate 207. Also, line 50 is connected inplace of line 48 to the second input of the AND gate 208. These reverseconnections can be achieved very simply by a four pole/double throwswitch (not shown) connected to accomplish this function.

In summary, the plugging circuit 9 causes the motor power supply 13 toapply a voltage so as to plug one of the stator windings when the motor10 is overspeed. The plugging circuit 9 responds to position pulses fromthe zero crossing detector 16 and to the state of the electronicstepping switch 14 and determines whether a zero crossing occurs priorto the time that it should occur. If the zero crossing is premature, anoverspeed condition is indicated, and the plugging circuit 9 controlsthe motor power supply 13 to apply voltage so as to plug one of thestator windings. Plugging results in retardation of the rotor rotationand consequent reduction in the rotor speed, thereby remedying theoverspeed condition.

By use of plugging, overspeed conditions which occur due to decrease orremoval of the load, increase in line voltage, or manual decrease of theselectable speed established by the rate variable oscillator 21 arecorrected. Moreover, oscillation of the rotor speed, due to overshootand undershoot of the selectable speed established by the rate variableoscillator 21 during startup or manual increase in the selectable speed,is prevented. Consequently, the use of plugging assures stable,synchronous operation of the motor 10 at a fixed selectable speed.

It has been found that the use of plugging can effectively supplant theneed for voltage amplitude control. Consequently, this inventioncontemplates the elimination of the voltage amplitude control circuit 26when the plugging circuit 9 is included. This may be accomplished byadjusting the magnitude of the voltage for the motor power supply 13 tothe desired value. By way of example, the movable contacts 73 and 74 inFIG. 5 would be positioned to corresponding taps on the secondarywindings 66 and 67, respectively. The taps would be selected so that thevoltage magnitude is sufficient to enable the motor 10 to attain anyspeed in a desired range of selectable speeds under expected maximumload. Furthermore, the connection between the amplitude control circuit26 and the motor power supply 13 in FIG. 4a is open-circuited as arelines 87 and 88 in FIG. 4. Also, the up and down terminals in FIG. 11are disconnected from the up and down inputs, respectively, to theactuator 75 in FIG. 5.

Where the motor 10 is expected to operate over a wide range of loads orspeeds, however, the voltage amplitude control circuit 26 is preferablyincluded. This allows the voltage pulse width to be maintained withinpredetermined limits so that the motor 10 runs smoothly. Otherwise, thevoltage pulse width may become too narrow under small loads or at lowspeed which tends to make the motor 10 unstable and leads to repeatedoperation of the plugging circuit 9, thereby causing the motor to surge.

The foregoing description of a preferred embodiment of the invention hasbeen made with particular emphasis on the block diagram shown in FIG. 4.The function of each element shown in the system diagram of FIG. 4 hasbeen fully explained and, in many instances, specific circuits have beendescribed for implementing these functions. For those circuits whichhave not specifically been described, numerous approaches forimplementing these functions will occur to those skilled in the art. Theexact implementation for the preferred embodiment of the controlcircuitry of the invention is shown in FIGS. 7-11. In these drawings,the exact circuit diagram is shown including specific circuits for atypical machine characterized by this invention. The only modificationswhich may be necessary to adapt the circuits there shown to a givenapplication would be to include voltage level shifting circuitry toaccommodate a machine whose stator windings were powered by signalshaving different voltages than that for which the circuit was designed.

Other Machine Configurations

While the foregoing description has been made with particular emphasison a machine having two stator windings, i.e., a two phase machine, asshown schematically in FIG. 1, the principles of this invention apply tomachines having different stator winding configurations. For example,three phase/four wire machines, of the type shown schematically in FIG.12 can also be controlled by systems generally of the type describedabove, although certain minor modifications are necessary because threestator windings are utilized rather than two.

The machine shown schematically in FIG. 12 is a three phase/four wiremachine which includes three stator windings labeled the A coil, the Bcoil, and the C coil. These coils are physically part of the stator,shown generally at 176, and are arranged so that the axis of eachmagnetic field is arranged at an angle of 120° to the axis of the fieldgenerated by the other two stator windings. The rotor, shown generallyat 177, is of the permanent magnetic type and has a flux associatedtherewith which interacts with the field generated by the statorwindings to cause rotation of the rotor 177.

One lead of each stator winding labeled the A coil, the B coil, and theC coil is available for connection to a motor power supply. The otherlead of each of these stator windings is connected to a common groundpoint indicated at 178.

Referring to FIG. 13, a sequence of power pulses for operating the motorshown schematically in FIG. 12 is shown. According to this sequence,current conducts in two stator windings at all times, providing there isno pulse shortening due to the action of a commutation circuit and apulse centering circuit. Specifically, the sequence of applying powerfor sustaining rotary motion in one direction for a three phase/fourwire machine is as follows: -B +C, +A -B, +A -C, +B -C, -A +B, and -A+C. By reversing this sequence, the machine can be made to rotate in theopposite direction.

Referring to FIG. 13, the zero crossings of the induced voltage in thestator windings, for the machine shown in FIG. 12 wherein the rotor isthere shown at its 90° position while rotating in a clockwise direction,occur when the rotor is positioned at 0°, 60°, 120°, 180°, 240°, 300°and 360°. As can be seen from the signals representing when power isapplied to the A, B, and C coils, these zero crossover points occur at atime displaced by the time required for the rotor to rotate 60°.Therefore, a counter arrangement like that described in connection withFIG. 4 can also be utilized in three phase machines to determine theposition of the rotor.

In order to permit a two counter system of the type shown in FIG. 4 togenerate position signals for a machine of the type shown in FIG. 12, asmall circuit modification like that shown in FIG. 14 is necessary. Eachof the stator windings in a three phase machine has associated therewitha zero crossing detector of the type shown in FIG. 4. The output wire ofeach of these zero crossing detectors produces a pulse whenever a zerocrossing is detected for the induced voltage in the stator windingconnected thereto. The zero crossing signals are presented to one inputof an OR gate 180, as shown in FIG. 14. The output of this OR gate 180is connected to one input of two AND gates 181 and 182. The second inputto the AND gate 181 is connected directly to the output of a flip-flop183 while the second input of the AND gate 182 is connected to theoutput of an inverter circuit 184 whose input is connected directly tothe output of the flip-flop 183. Consequently, the second input to theAND gate 181 will always have a signal applied thereto having a logiclevel opposite that of the second input to the AND gate 182. Thus, apulse generated at the output of the OR gate 180 indicating thedetection of a zero crossing of induced voltage in one stator windingwill pass through either the AND gate 181 or the AND gate 182 but notboth. The output of the AND gate 181 is connected into the circuit ofFIG. 4 and corresponds to the input wire 116 to the commutation circuit17. The output of the AND gate 182, on the other hand, corresponds tothe input wire 122 to the commutation circuit 17. The signals on thesewires 116 and 122 serve the same identical functions as previouslydescribed.

Each output of the AND gates 181, 182 are connected to one input ofanother OR gate 185. The output of this OR gate 185 is connected to theinput of a delay line or similar delaying circuit 186 whose outputapears at a time delayed from the time when the input is appliedthereto. This delay 186 is connected to the input of the flip-flop 183.As such, each time a zero crossing is detected, this zero crossingdetection is operational to generate a pulse on either wire 116 or 122.This pulse is also operational to change the state of the flip-flop 183so that a subsequent zero crossing detected will reverse the line 116 or122 on which the pulse appears. Therefore, subsequent zero crossingsdetected by the zero crossing detectors are operational to alternatelygenerate pulses on the wires 116 and 122. These alternate pulses areutilized in a manner as described earlier in connection with thecounters 18 and 19 of FIG. 4.

In addition to the modifications shown in FIG. 14, the electronicstepping switch 14 of FIG. 4 must be modified to generate six uniqueoutput signals in sequence in response to the input pulses from a startcircuit or a commutation circuit rather than the four shown for thecircuit in FIG. 4. The motor power supply 13 in FIG. 4 must also bemodified to utilize these six outut signals to generate six power pulsesaccording to the sequence described above in connection with the motorshown generally in FIG. 12. Additionally, the plugging circuit 9 of FIG.4 must be expanded to include six AND gate/latch tandems rather thanfour so as to sense during each rotor rotation premature occurrence ofany of the six zero crossings of voltage induced across the statorwindings. The outputs of the six latches connect along with the sixoutputs of the modified electronic stepping switch through six OR gatesto the six inputs of the modified motor power supply. This will assurethat the three phase motor in FIG. 12 will be synchronized at theselectable speed established by a rate variable oscillator whosefrequency is such that six evenly spaced pulses are generated during theperiod of time the rotor completes one turn at the selected speed ofrotation.

Modification of the zero crossing detectors may also be necessary in amachine having three or more phases, especially where the machine isoperated over a wide range of loads and/or synchronous speeds or, ifsynchronous operation is not desired, the speed is allowed to vary withchange in load. This is in consequence of transformer action occurringin machines having three or more phases since such machines have statorwindings that are physically oriented with respect to each other at anangle different than 90°. The three phase machine in FIG. 12, forexample, has stator windings oriented with respect to each other at anangle of 120°.

When the voltage across a stator winding changes, the magnetic field inthe stator winding changes. Since the stator windings in a three or morephase machine are oriented with respect to each other at an angledifferent than 90°, the changing magnetic field produces a voltage bytransformer action across other stator windings. Changes in voltageacross a stator winding are primarily due to switching transientsoccurring when voltage is applied to or removed from the stator winding,although changes in voltage across a stator winding are also influencedby voltages induced across the stator windings by the rotating rotormagnetic field and the interaction of the induced voltages with thevoltage that is applied to powered stator windings.

The characteristic of the voltage induced by transformer action dependson the load and machine speed. If the machine is maintained at aselectable speed when the load changes, the duration and/or amplitude ofvoltage pulses which are applied to the stator windings is adjusted tomaintain the selectable speed. This affects the characteristic of thevoltage due to transformer coupling among the stator windings. If themachine speed is allowed to vary with change in load, that is, the curveto the right of T_(m) in FIG. 3 characterizes machine operation, theduration and/or amplitude of voltage pulses which are applied to thestator windings is not adjusted, but the machine speed changes. Thisaffects the characteristic of the voltage due to transformer couplingamong the stator windings.

As shown in FIG. 6, if the machine is maintained at a selectable speedwhen the load changes, the curve +V', 94', 95, 96, 97, 98, 99 representsthe voltage across a stator winding at a lesser load. Curve +V, 94, 95,96', 97', 98', 99 represents the voltage across a stator winding at agreater load if one assumes that the voltage pulse duration is maximumand voltage amplitude control is used to maintain the selectable speed.Although the zero crossing 99 occurs at the same time since theselectable speed is maintained, the zero crossing 97 for operation atthe lesser load occurs sooner than the zero crossing 97' for operationat the greater load.

If the machine speed is allowed to vary with change in load, at a higherload, the machine speed is lower, and the curve +V, 94, 95, 96', 97',98', 99' represents the voltage across a stator winding. At a lowerload, the machine speed is higher, and the curve +V, 94, 95, 96, 97, 98,99 represents the voltage across a stator winding. The zero crossing 97'for operation at a higher load, or lower speed, occurs later than thezero crossing 97 for operation at a lower load, or higher speed.

As shown in FIG. 6, the time during which the voltage across the statorwinding passes through the range between -V volts and zero volts varieswith change in load and/or machine speed. The exact time at which thevoltage crosses through zero volts, such as at 97 or 97', can varysignificantly in a machine with three or more phases, especially wherethe machine is operated over a wide range of loads and/or synchronousspeeds or, if synchronous operation is not desired, the speed is allowedto vary with change in load. Consequently, modifying the period of thedelay 105 in FIG. 4 might not accomplish the desired objective ofindicating only zero crossings of voltage induced across a statorwinding, such as at 99 or 99', if the three or more phase machine isoperated at different loads and/or speeds. That is, if the machine ismaintained at a selectable speed when the load changes, a delay 105which would have a period sufficient to cause the zero crossing 97' tobe ignored when the load was high might be sufficient to cause the zerocrossing 99 to be ignored when the load was low. Similarly, if themachine speed is allowed to vary with change in load, a delay 105 whichwould have a period sufficient to cause the zero crossing 97' to beignored at low speed might be sufficient to cause the zero crossing 99to be ignored at high speed.

With reference to FIG. 6, the slope of the voltage is positive at zerocrossings 97 and 97'. The slope of the voltage is negative at zerocrossings 99 and 99'. In accordance with the present invention,indication of only zero crossings 99 or 99' of the voltage inducedacross the stator windings by the rotating rotor magnetic field isachieved by modification of the zero crossing detectors to include slopesensors.

A slope sensor 299 is shown in FIG. 18. The slope sensor 299 includes adifferentiator 300. The inverting input of an ideal differentialamplifier is connected through an input resistor and a capacitor to astator winding at input terminal 301. The non-inverting input isconnected through a resistor to ground. The differentiator 300 alsoincludes a negative feedback resistor. The voltage at the outputterminal 302 of the differentiator 300 will be negative when a positivegoing voltage appears across a stator winding and positive when anegative going voltage appears across a stator winding.

The output terminal 302 of the differentiator 300 is also connectedthrough an input resistor to the inverting input of an idealdifferential amplifier 303 which functions as an inverter. Thenon-inverting input of the inverter 303 is connected through a resistorto ground. The voltage at the output terminal 304 of the inverter 303will be positive when a positive going voltage appears across a statorwinding and negative when a negative going voltage appears across astator winding.

In a three-or-more phase machine, the zero crossing detector for eachstator winding is modified to include a slope sensor 299 such as shownin FIG. 18. By way of example, and with reference jointly to FIGS. 4 and18, the input terminal 301 is connected to a stator winding, such as atterminal 89. The output terminal 304 is connected to the input of an ANDgate corresponding to the AND gate 111. The output terminal 302 isconnected to the input of an AND gate corresponding to the AND gate 112.Slope sensors are similarly connected in the zero crossing detectors forthe other stator windings.

The modified zero crossing detectors operate to produce an indicationonly when a zero crossing of the voltage induced across a stator windingby the rotating rotor magnetic field occurs. By way of example, supposethe voltage in FIG. 6 appears across the A coil in a three phasemachine. If the zero crossing detector were modified as described above,the output of the AND gate 112 is positive whenever the voltage inducedacross the A coil is negative, the signal on the wire 59 is positiveindicating that a positive power pulse is being applied to the B coil,the induced voltage in the A coil has been greater than -V volts for theperiod of the delay 105, and the output terminal 302 of the slope sensor299 in FIG. 18 is positive indicating that the induced voltage in the Acoil is negative going. This condition occurs just after the inducedvoltage across the A coil goes through zero volts at 99 or 99' in FIG.6. This would correspond to the zero crossing of the voltage inducedacross the A coil occurring at 180° in FIG. 13.

The delay 105 is included in the zero crossing detector to prevent apositive output from the delay line 104 during the time that the voltagein FIG. 6 crosses through zero volts at 95 and the time that the voltagecrosses through -V volts going toward -4 V volts. This preventsindication of the zero crossing 95. The slope sensor 299 then preventsindication of the zero crossing at 97 or 97'. Consequently, only thezero crossing 99 or 99' is indicated. The addition of the slope sensorto the zero crossing detectors for the stator windings of a machinehaving three or more phases makes possible indication of only zerocrossing 99 or 99' no matter what the machine speed is.

When a machine having three or more phases is operated in a reversedirection, the connection of the output terminals 302 and 304 of theslope sensors must be switched. That is, the output terminal 304 must beconnected to the input of the AND gate to which the output terminal 302was connected and vice versa in each zero crossing detector when themode of operation is changed from forward to reverse.

It will be recognized by those skilled in the machine art that thetechniques of pulse widening and shortening described in connection withFIG. 4 are equally operative in connection with machines of the typeshown in FIG. 12. The principles of the invention may also be applied inthe operation of three phase/three wire motors.

Referring again to FIG. 13 it is possible to structure an Up/Downcounter arrangement so that there is a pair of counters associated witheach of the stator windings. The A counters, B counters, and C countersare easily activated in response to zero crossings detected for theinduced voltage in the A coil, B coil, and C coil so that a timingsequence, like that shown in FIG. 13, can be generated. This timingsequence will generate borrow or second position pulses from either theA, B or C counter at times when the rotor is at a known position. Theadvantage achieved by having pairs of counters associated with eachstator winding is that it is possible, with this configuration, to powerthe stator windings for a different length of time than that shown forthe power pulses in FIG. 13, thus permitting the machine performance tobe selectively modified.

It will be clear to those of skill in the art that the principles of theinvention as outlined above in connection with two phase and three phasemachines may easily be extended to machines having any number of phases.Indeed, any machine of the type herein described with any number ofphases and any number of poles can be operated in accordance with theprinciples of this invention.

It will be readily recognized by those of skill in the art that theforegoing description has been made with particular emphasis on apreferred implementation shown in the drawings. The specificimplementation shown in the drawings however, is merely representativeof a preferred implementation for the functions described. Thisdescription is not, however, intended to be exemplary of every possiblelogic configuration for performing the functions defined. Indeed, manycircuit modifications may be made to the specific configuration shown inFIGS. 7-11 while still maintaining the same function as described inconnection with FIG. 4. Further, those skilled in the art will readilyrecognize that the described functions and their equivalents can beimplemented in MOS technology which is advantageous because assemblycosts can be dramatically reduced. It will also be recognized by thoseskilled in the art that these and other modifications in form only maybe made without departing from the spirit and scope of this invention asdefined more particularly by the following claims.

Instead of hard wired logic as illustrated herein, the motor can also becomputer controlled as, for example, by the use of microprocessors whichare programmed to perform the basic functions described herein.

What is claimed is:
 1. In a three-or-more phase machine with a statorhaving a plurality of windings thereon, each stator winding producing amagnetic field when voltage is applied thereto, a rotor rotatablymounted with respect to said stator, said rotor having a magnetic fieldtherein fixedly oriented with respect to said rotor, the rotor fieldbeing operative to induce a voltage in the stator windings when therotor is rotating, and a power circuit for applying positive andnegative voltage to the stator windings, switching transients bengproduced when voltage is applied to and removed from a stator winding, arotor position sensing circuit, comprising:a voltage detector connectedacross a stator winding, said voltage detector being operative only whenthe voltage across the connected stator winding is more positive thanthe amplitude of a negative voltage following the turn off of a positivevoltage applied to said stator winding, said voltage detector producinga polarity signal when voltage across the connected stator winding ismore positive than the amplitude of said negative voltage; a delay meansresponsive to said polarity signal to produce a delay polarity signal,said delay being at least as long as the time required for negativegoing switching transients across said connected stator winding to passthrough the voltage range between the amplitude of said positive appliedvoltage and the amplitude of said negative voltage; a first gate meansresponsive to said polarity signal and said delayed polarity signal toproduce a transient completed signal; a polarity detector connected tosaid stator winding for producing a sense signal when the voltage acrosssaid connected stator winding is negative; a slope sensor responsive tothe voltage across said connected stator winding for producing an enablesignal when said voltage across said connected stator winding isdecreasing; and a second gate means responsive to said transientcompleted signal, said sense signal, and said enable signal to produce areference position signal indicating that the rotor has a knownpositional relationship with respect to the stator.
 2. In athree-or-more phase machine with a stator having a plurality of windingsthereon, each stator winding producing a flux when voltage is appliedthereto, a rotor rotatably mounted with respect to the stator, saidrotor having a magnetic field therein fixedly oriented with respect tosaid rotor, the rotor field being operative to induce a voltage in thestator windings when said rotor is rotating, and a power circuit forapplying positive and negative voltage to the stator windings, switchingtransients being produced when voltage is applied to and removed from astator winding, a rotor position sensing circuit comprising:a detectormeans connected to each stator winding for producing a zero crossingsignal when the voltage across the connected stator winding passesthrough zero; and means including a slope sensor responsive to each saiddetector means for blocking said zero crossing signal when said zerocrossing signal is caused by a switching transient zero crossing and forpassing all other said zero crossing signals as a rotor position signalindicating that the rotor has a predetermined positional relationshipwith respect to the stator.